Frankly, I have never tried this application with three resistors VGA monitor driving by an FPGA. But it seems reasonable.. Should be.. At the other hand, this site is the one of best about FPGAs on the web that I think.. It worths to try.. Almost zero risk..
the pong game on FPGA4fun uses a 3bpp (bits per pixel) vga monitor core. if you need to show an 8bpp image on the monitors you will need to design an 8bpp core and you will need an 8-bit triple DAC like the THS8134B (the XSB-300E board from XESS uses a simillar DAC).
what do you want to display on the monitors? text? graphics?
Agree with SamCheetah, it depends on ur application, if it require variety in colours than u can use 8bit or 10bit triple video DAC, just u have to take care in generation of sync signal.