3-prong tamper proof screw type?

These are indeed very rare. And old. Google image search kept showing old radio sets.

After a lengthy search I found one source that should do the trick for you.

Microtech Medium Domed Tri-Wing Wrench Tool Bit (bladehq.com)

Here's what I eventually did to get the screws out:

• Used a Dremel rotary tool with a metal cutting wheel on it
• Cut slots across the head of each screw
• Used a flathead screwdriver to remove them

Hope that helps anyone who might come across this issue in the future.