Make Solid-state scrolling trackpad behave like Multi-Touch trackpad
Solution 1:
You could try with some software like Better Touch Tool, but that's not guarantied it will work, since it's more of a hardware limitation.
Another solution would be to buy a Magic Trackpad.
Solution 2:
Sadly, there is no way and no app to add this functionality.
The hardware simply wasn't designed to track two (or more) independent touches. You get two finger (wide contact point) scroll but that's it.
You would need to add more hardware like a video camera to see three fingers and analyze the movement optically (or add expensive circuitry and somehow hook into a free port like bluetooth, USB and add a kernel extension to get that added input to the Human Interface system.) It's more practical to find a buyer that wants your mac and put that money towards a mac with true multitouch hardware than to add multitouch capability after the fact.