guitars

Working on an unholy abomination of a diy instrument and I'm planning on using the hardware from an electric guitar I got for free at a yard sale. I already took a deep dive into pickup construction and wiring, but the explanation for where they put them on the guitar mostly in guitar player jargon and I'm not sure if that translates to this.

Basic questions, why are there 3 pickups? Why is one behind where you strum and the other two up front? Is there an ideal placement for different sounds?

Any help would be appreciated, I'm not much of a musician, and this is really just a silly project just to see if I can do it but anything worth doing is worth overdoing and I like the chance to learn.