this is the 2009 and newer forum. This should really be in the 97-03 forum. But to answer your question, #3 is on the passenger side, third one back. For future reference, when you open the hood and are looking at the motor (on a V-8) the cylinder closest to you on your left is #1. Behind that is #2, 3 and 4. On the right is #5, 6, 7 and 8. Pretty simple. The same strategy holds true for v-6 and v-10s as well. Inline motors are pretty self explanatory. #1 in front and go back from there.
I assume you are asking this question because you fouled a plug or coil on plug. If so, remember to put anti-sieze on the new plug threads. If the plug doesn't want to thread, don't force it. You have aluminum heads and they can cross thread easily. If the motor shot the plug out and destroyed the coil on plug, you need to take the truck in and have a professional install a helicoil.