TracyT4i wrote:
100mm f2.8L
You don't mention whether you have a full frame or crop camera, and that's pretty important.
For example, I'll assume you have a crop camera (e.g. Nikon D7100).
Three people sitting in leaves will need at least 6' x 4' of space. You don't want them all lined up, but staggered some. Let's give them each 10 inches, so you need 30 inches (2.5 feet) front to back. Now you can focus on the middle person, so you need 20 inches to the back and 10 inches in front. DOF usually gives more to the back than front. So the key number is 20 inches (1.65 feet). You'll get the front person more in focus that way, as well.
With a 100mm lens, you need to be at a distance of 27 feet to get a 6' x 4' field of view (FOV). At 27 feet, the DOF is 1.3 feet behind the subject at f/2.8. You can move back to about 30 feet to get the desired DOF (increasing the FOV to 6'9" wide), or you can decrease aperture to f/3.6.
For me, if I was interested in narrow DOF, I would shoot at 27 feet with an aperture bracket of +/-2/3EV (in other words, shoot at f/2.8, f/3.6, f/4.5) and then pick the shot I liked.
If your camera will automatically shoot an aperture bracket, then set the bracket to 3 frames/0.7EV, set camera mode to shutter priority, burst mode, exposure compensation to -2/3EV (to avoid overexposure), and adjust shutter speed and ISO to get an aperture of f/3.6 (with shutter speed at least 1/160). And shoot RAW, because the shots will vary by 4/3 EV. Then fire away.
Otherwise, you can set to aperture mode and adjust aperture as you shoot. In that case, lock the camera on a tripod so the framing stays the same as you rotate aperture and press shutter (hard to keep the camera pointed consistently when doing this).