you can do what you want with cpu miners but not gpu. i can't even watch videos on the internet or my computer while i am mining because i underclock my VRAM and overclock the GPU.
with 3 fast cards, you will pull nearly a GH/s when you are mining, so i wouldn't worry about a couple of hours a day where you shut them down for some gaming.
guiminer/poclbm try these settings -a 5 -w256 BFI_INT AGGRESSION=19 -f 0 VECTORS see if you don't start getting more share submits. I get 105+ an hour with my single 5850!
Underclock your ram to 300Mhz, or 900mhz. try setting AGGRESSION to 13, as well. and -f 1. futz around and tweak till your happy with your share output.