All three have different fits. The 703 is narrow all the way across, the 704 (best I can tell the same as a 735) is narrow at the heel and has more depth/room up front, the 705 is medium spaced all the way across.
Get a good fit before you buy skates, especially Grafs. Make sure the heel locks into the skate and doesn't move around, and then make sure the boot is comfortable yet snug.
My understanding is that the 703 replaced the 704 and they are basically the same skate, except the 703 has the updated synthetic liner materials while the 704 had the older materials. I've skated on both of these and they fit me identically.