Install, then go the crysis 3 folder, open system.cfg and change r_Driver=DX9 then save it, Proof Here http://www.crydev.net/viewtopic.php?f=326&t=89555

Will this enable crysis 3 to run on DX9?

no, that's for the sandbox editor. Crysis 3 the game doesn't support a DX9 renderer.