How can I set Windows 7 up to use three monitors?

Windows 7 does not allow use of onboard graphics at the same time as a video card without Hydra (which you motherboard does not support).

I asked a similar question: Is it better to add a second video card in Crossfire or replace an aging video card?.

It looks like your motherboard does not support SLI/Crossfire. You're stuck having to buy a new graphics card. Sorry.

UPDATE

You will need to buy a graphics card that has three outputs which are generally a dual PCI slot cards. Unless you get a card that has displayPort in it.

I just started to use a eVGA UV Plus USB to DVI adapter at work which is driving a Dell 19" standard monitor, this is in addition to dual Dell 22" wide screens. We'd purchased them for dual setups on some older Dell USFF machines which lack any expansion.

Win 7 x64 found and installed the driver when I plugged in the unit, and it provides surprisingly good output.