Is there an adapter for a TO-220 part to go into a TO-3
I'm pretty sure I've seen this before: Just cut the middle leg off the TO-220, bend the remaining leads 90&deg; at the right distance, and mount the TO-220 to one of the existing TO-3 mounting holes. It isn't hard to get the two remaining TO-220 legs to to line up with the TO-3 pins reasonably well.