You will get a lot of answers here depending on what cameras people have owned and used. If you have friends who do photography, I would get the same camera they use. Otherwise, I would probably argue for the Canon sl2 or the Nikon 3400 (or new 3500). The sl2 is a nifty little camera that produces good results; Canon offers a pretty good set of lenses for their crop-sensor cameras and their L lenses work well on the body. I haven't used the 3400, but I did have a 3200, which was a great little camera with lots of lens choice like the Canon. Both have instructor guides built into the camera if you need/want it. Pentex makes a great camera--you can't go wrong there either, but Pentax doesn't have the lens selection Canon and Nikon have. Basically, you can't go wrong with any of them.

I don't know much about the Nikon D3400 or the Pentax K-70...

Between the two Canon models, I'd definitely go with the SL2 over the T7.... The T7 is the most entry level Canon of all and lacks a lot of things. The SL2 is the world's second smallest and lightest DSLR (only because the preceding SL1 is slightly smaller and lighter), but in spite of that the SL2 has more features than the T7. In fact, the T7 is nearly identical to its predecessor.... it got a sensor upgrade (18MP to 24MP), but otherwise is pretty much the same as the T6. It even uses the same old Digic 4 processor (Canon is now putting 2 and 3 generation newer processors in their other cameras). The SL2 has a three generation newer Digic 7 processor that allows for much better performance in a number of ways. In fact, the T7 is very difficult to find... I couldn't find it at Amazon, B&H, Adorama, Costco or Walmart. The only place I see it offered is at the Canon USA website.

SL2 has an articulated, 1 million dot, Touch Screen LCD.... T7's is fixed, 920K dot, and is not a Touch Screen.

SL2 has ISO 100-25600 range (expandable to 51200).... T7 has the same ISO 100-6400 (expand: 12800) range as its predecessor.

SL2 can shoot continuously at 5 frames per second.... T7 can only shoot at 3 frames per second.

SL2 can take advantage of higher performance UHS-1 SD cards.... T7 can use them, but doesn't support UHS-1.

Both cameras have built-in WiFi, but SL2's is Bluetooth/Smartphone capable while T7's is not.

SL2 is rated for 30% more shots per battery charge.

SL2 has a self-cleaning sensor, like most Canon DSLRs.... T7 is one of the few that doesn't. (I will never own another DSLR without this feature.)

SL2 has Dual Pixel AF in Live View (" DPAF" was introduced on the 70D and is on most Canon ever since... T7 is one of the last Canon that's still using much slower contrast detection AF in Live View.

Both cameras use essentially the same 24MP sensor.

Both cameras use a similar 9 point AF system with a single, higher performance "cross type" point at the center. The rest of the points are single axis, a mix of vertical and horizontal.

Neither camera has option to fit a battery grip to be able to use a 2nd battery or give vertical controls.

When bought in 2 lens kit, SL2 is usually bundled with EF-S 18-55mm IS STM and EF-S 55-250mm IS STM lenses.... This bundle is currently on sale many places for $750.

I only found T7 bundled with EF-S 18-55mm IS II lens, which is a little cheaper, slower and noisier focusing than the STM version. Predecessors were also often bundled with Canon's cheapest tele-zoom, the EF 75-300mm "III". The EF-S 55-250mm IS STM is a bit more expensive but a MUCH better lens in all respects: It's got noticeably better image quality than the 75-300 III. It also is closer focusing, faster focusing, quieter focusing. And the 55-250mm is image stabilized, which the 75-300 III lacks. Watch out for the 75-300 III bundled with other Canon cameras, and avoid it if at all possible.

Speaking of which, you might want to consider some other Canon models. For example, the T6i and T6s are both also 24MP and more up-scale than either SL2 or T7.... and are widely available at good prices. T6i/T6s have a better 19-point, all cross type AF system (much like the 70D's). These models can be found significantly discounted, because they have been superseded by T7i and 77D (45-point AF system, similar to 80D's). The T6i/T6s both can optionally be fitted with BG-E18 battery grip (no grip has been offered for the newer models). The T6i/T6s do not have DPAF, but have most of the other features of SL2, and more.

Canon USA sells refurbished cameras directly from their website. These have the same warranty as new... and buyers report are often little different from new. Right now they have refurbished T6i bundled with EF-S 18-55mm IS STM and EF-S 55-250mm IS STM lenses for $629 (plus sales tax... free shipping). Or, T6s is available with the same lenses for $729. (The main differences between T6i and T6s are primarily that the latter has more buttons and dials for direct control of various functions, plus has additional display on its right hand shoulder that shows most camera settings at a glance.)

You can do general comparison of cameras pretty easily using this website:
https://cameradecision.com/compare/Canon-EOS-Rebel-T7-vs-Canon-EOS-Rebel-SL2
https://cameradecision.com/compare/Canon-EOS-Rebel-T6i-vs-Canon-EOS-Rebel-SL2
https://cameradecision.com/compare/Canon-EOS-Rebel-T6i-vs-Nikon-D3400
https://cameradecision.com/compare/Pentax-K-70-vs-Nikon-D3400

There are a couple other websites that do similar comparisons... Google them if you wish.

While I don't know a great deal about those specific Nikon and Pentax models, I can tell you that Canon and Nikon offer considerably larger selection of lenses and accessories than Pentax. Both Nikon and Pentax have some backward compatibility with their respective earlier, manual focus lenses.... but both vintage Nikon F-mount and Pentax K-mount can also be used on Canon cameras via simple, inexpensive adapters.

Canon and Nikon use in-lens image stabilization. Pentax uses in-camera stabilization, which has advantage that any and all lenses used on the camera benefit from it. OTOH, with DSLRs like these, in-lens stabilization also serves to stabilize what's seen in the viewfinder. That's not the case with in-camera stabilization like the Pentax use.

In general, I've noticed with the initial purchase that Nikon camera & lens kits tend to be a little less expensive than comparable Canon. However, after that you will find that any Nikon lenses and accessories you might want to add in the future tend to be a little to a lot more expensive than comparable Canon items. I haven't compared the cost of Pentax kits or separate items.

If ever needed, Canon's warranty repair and customer service usually get very high marks. Nikon is pretty good, too.... though they are much more restrictive. They control all the costs and won't sell parts to anyone outside their own network of licensed repairers, which is about 20 shops nationwide in the U.S. In comparison, anyone can call up Canon and purchase a part from them, if needed for repair. That means there are literally hundreds of shops that can perform work on Canon gear, if needed. I have no idea how Pentax compares.

If possible, go handle the cameras in a store to see if you have any preference. Hopefully you'll be able to turn the camera on and explore the menu, too.... see if it seems easy and intuitive to navigate. Canon menus generally are considered among the easiest... but I haven't compared them personally and I'm sure any of them can be learned. I've heard more complaints about Sony menus... not much at all, one way or another, about Pentax.

Worldwide, during the first part of 2018 interchangeable lens system camera (combining DSLRs and mirrorless) market share was reported August 1: 49.1% Canon, 24.9% Nikon, 13.3% Sony. Those top three have 87.3% of the market. The rest of the manufacturers (Pentax, Olympus, Leica, Panasonic, Fujifilm, etc.) split the remaining 12.7% among them.

Have fun shopping!

Of the choices available, I'd definitely recommend the Pentax k-70. Reasons: in-body stabilization, weather sealed, incredibly sturdy, good selection of native and 3rd party lenses, articulating lcd with night vision, and pixel-shift for high resolution. The pixel shift is supplemented by motion correction which throws away the extra 3 shots for the area where motion is detected and builds that part of the image the same as it would have been without pixel shift. Nothing is perfect, but this is pretty darn close.