1 - under the front of the trigger plate
2 - under the back of the trigger plate
3 - behind the trigger plate
The two that are underneath the trigger take up the extra vertical slack between the frame and the body, and prevent the trigger plate from rotating in the frame and binding againt the frame or body.
The one behind the trigger plate also sits directly behind the trigger return spring, and can act as a "tensioner" for the return spring - in other words, turning the screw in will increase the starting tension on the spring, turning the screw out will reduce the starting tension on the spring, thus making the trigger softer or stiffer.
You can also buy (or make your own) a positive trigger stop from G-Force (I believe that this is available at G3) which will provide a physical stop for the trigger if you use it in place othe the third set screw.
P.S. - This is all based on what's in _my_ P&P frame yours may or may not be different...