Will the nikon SB 700 flash work with my d3100? Nikon only list the 600 and the 900?

Want to make sure it will sync and all in all modes and not just in M mode. Thanks!

Added (1). I mean that I want to make sure that my camera will see that there's a flash on it… And not try to have the other one pop up.

Yes it will

Yes it will. It's an older Nikon flash but any of the Nikon SB flashes will fully operate with all Nikon DSLRs and it will work perfectly fine with the Nikon CLS system.

On page 173 there's a list of all the Nikon SB flash units that are compatible with the CLS system your camera uses.

On page 174, there's a list of other compatible flash units you camera can use.

Why do you think that Nikon only lists the SB-600 and SB-900?

What do you mean, "Want to make sure it will sync and all in all modes and not just in M mode"?

Your camera only flash syncs at 1/200th or slower shutter speeds, so no matter what mode you are shooting in, the shutter speed can't exceed that upper limit.

You may want to read the section in your user manual on how to use your external flash on pages 68 to 70