1) Get a Quad-Core if you wanna run more than 3 on the same PC; I have a dual-core too and 2 runs smoothly, 3 runs mostly smoothly though you can tell it's starting to push the limits, 4 will be like giving a fat guy a piggyback ride, "she cannae take any more captain!" :p
2) Attempt to limit the FPS on the background clients as described by keyclone, I forget the exact command but it was something like /console maxfpsbk 15/20/etc; limiting the FPS on the background clients may improve the front copy and you will likely not notice much difference in the background ones, but your poor stressed processor will lol
3) If all else fails, downgrade to 4(1 on off PC, 3 on main) and you'll still be able to run instances for the most part on your own; down to 3 if you have to but again your options above would solve the numbers issues if you can afford it