Should I trade Devonta Freeman?

Devonta Freeman is the best player in fantasy through the first four weeks, and there is no reason to think he won't continue to be heavily used by the Falcons. It's impossible to sustain his current production level (he's not going to continue to score 3 TDs every week) but he is solid along the lines of Le'Veon Bell or Adrian Peterson.

Why not trade Foster (big question mark) or McCoy (can't stay healthy and being outplayed by Karlos Williams)?

With Freeman and Gurley, you're set at RB with young studs...I wouldn't trade either of them. You've got to be a strong contender for your league championship.

Yes, you should if you can get a good deal. It's unlikely that Freeman will continue to get 3 TDs every week and Coleman will get some of the work when he's healthy