My HB and I spent 10 days travelling around Iceland at the beginning of July. Needless to say, the trip really exceeded our expectations. There are countless photo opportunities wherever you go, even while driving along the Ring Road - you feel like you want to keep stopping to shoot at the beautiful landscape. We also took advantage of the very long days for good photo opportunities, as the sun barely sets in the horizon during the summer months. It is rather unfortunate that you are only spending 3 days in Iceland, but try to make the best of it. You might get lucky and be able to watch the northern lights.
There are a wide variety of day tours leaving from Reykjavik that you can book, and some of those may be seasonal. You may just be constraint to the reduced daylight in October as the days will be shorter, depending on the time of your trip in October. We booked our self-drive tour with Nordic Visitor, they have lots of great information in their website; here is the link to information about Time and Daylight, :
https://iceland.nordicvisitor.com/travel-guide/information/time-daylight/A must-do tour is the Golden Circle tour which can easily be done in one day (do a bit of Google research and you will get plenty of information). As another poster mentioned, another must-see site is the Jökulsárlón Glacial Lagoon and adjacent Diamond Beach in southeast Iceland, but would require an entire day for a round-trip road trip to the lagoon from Reykjavik.
It looks like your photography gear should be more than sufficient for your photo needs.
While in Reykjavik, take time to visit the Harpa Concert Hall and also go to the viewing deck at the landmark church in the city (Hallgrímskirkja) for great views of the city.
The Blue Lagoon is a wonderful site to visit, especially if you visit as soon as you land in Iceland (to help recover from jet lag) or in the day of your departure, just to relax in the thermal waters before the long flight back home. Should you plan to go to the Blue Lagoon make sure to book tickets in advance (their website has all the information you will need and more!).
You can find wonderful examples of Iceland photos and Icelandic landscapes in Flickr groups such as "Iceland - Landscape" and "Beautiful Iceland".
Looks like you are all set with your camera and lens gear, that should be sufficient for your short trip. Enjoy it to the fullest!