If you drive dual-link DVI then you would need 2 for the 3 monitors using DVI.
If you use DisplayPort then 1 would be enough.
The 5750 can drive:
4 TMDS Signals (1 Dual-Link DVI/HDMI uses 2)
So you can drive 2 DVI/HDMI's & a DisplayPort.
If you use a DVI->VGA adapter then 1 card will be enough.