Any suggestions for what the best method of doing this may be? Could I just strip one end off a quick disconnect wire and solder it into the metal bracket of another to make a doubled connector?
I would be putting diodes in to isolate the lines. Soldering wires together should work fine.
If the PCB is common ground: wire all inputs on one side of the switch and wire ground to the other side
If you did that to make a button to press A, B, and C with one button, then the signal lines for A, B, and C would all be connected together. If you pressed the ‘A’ button, all three would activate. If you pressed the ‘B’ button, all three would activate, etc. That’s why Rufus mentioned the diodes. You could make a separate A+B+C button, without affecting your A button, B button, or C button, but only if you used diodes to keep the lines from pulling each other down.
(Assumes common ground. If its not common ground, whole 'nuther can of worms with no easy answer.)
How about relays? That could be 'teh awesum - every time you hit a button - click-bzzzt-clank… Tubes could be fun too: “No it’s not a button check, I’m just waiting for my stick to warm up.”