RE: Up in smoke!
You were using a battery holder with a fullsize connector that prevents reverse polarity by accident. All DC powered electronics will fry instantly if the polarity is reversed.
4 X 1.5 volt dry cells will work but Nicad or NiMh will require 5 cells to get the required 6 volts the Fasst receivers must have. If it even is a fasst system as the 3pm is also made in FM. I would still run it on 6 volt either way. Make sure you don't have a servo connected with a dead short in it or you might fry the next new one. Do the troubleshooting before you connect another. They don't do that too often for no reason. The switch wasn't altered and the wires reversed? Just trying to help you with some things to check.
Daniel