Try everything on High with 2s SMAA. If it's fast enough, try ultra with 2x SMAA.
Per Tom's review/benching of Crysis 3, the 2x SMAA setting is the sweet spot for performance + appearance quality. Based on personal experience, I agree.
With 6300:-
System Specs:High(with 2X MSAA) and if there's draw distance and terrain quality options bring those down to high from very high(actually i have forgotten the settings options of the game since i have played it almost a year before)
Depends what resolution you're on and what frame rate target you're looking for. At 1080p, a good mix of medium/high with FXAA should give you near 60FPS.