You can probably use three displays with the 6750 if you use a DVI to VGA/D-Sub adapter. It can't handle three displays that use the same signaling and HDMI and DVD-D use the same signalling, so you need to use the DVI-A signalling (the same used by the VGA port) to use three displays with that card.
Also, Intel's website says that the i5-750 doesn't have an IGP, so you probably don't have any integrated graphics anyway. The motherboard supports processors that have them, so it has ports for it, but the i5-750 doesn't have an IGP, so to use the integrated graphics ports, you'd probably have to downgrade to an i3. i5s and i7s didn't get integrated graphics until the LGA 1155 socket boards and even then, some of the i5s don't have IGPs.