I'm a rookie when it comes to electronics, but I was wondering, if you have a 12kV rated capacitor and then you charge it with a 3 kV power supply, does the capacitor discharge at the rated voltage of 12kV or does it discharge at 3kV becasue that was what it was charged with? This is prolly a dumb question, but thanks for any info.

When discharging the maximum voltage will be 3kV (the voltage you charged it with), and the voltage will drop quickly the longer you discharge.

Technically it`ll be slightly less than 3Kv as well, as a capacitor never fully charges.

Depends how impatient you are too, the longer you wait, the more charge you get, but it never quite gets full. Like if you cross half the room, then cross half the distance remaining, then half that, then half that... you never actually get to the other wall.