I'll need a plate to mount those three pieces to so I'm going to use more aluminum for this. I machined this piece down to 1/4" thick, and the size of the plate is 9" X 6".
 

Now you can see how this will work because If you look close you'll see the top of the flathead screws. This is so each block can turn once the key is inserted.
 

These are 1/8" diameter roll pins that I'll be using as 'stops' for each block. I used my Dremel to cut each long one in half. Now these pins are slightly larger than 1/8" diameter, meaning they are a press fit if you use a 1/8" diameter drill. As you insert the pin into your hole, the pin squeezes down a small amount which keeps it from going anywhere. Each pin that I cut is about 1/2" long.
 

Here you can see how the pins look a little better. I also added one on the other side of each block so the blocks stop once they've been turned 180 degrees. These roll pins are heat treated and should last for many years without any problems.

I mentioned earlier that once all three keys were inserted and turned that it would activate an electrical circuit. Well now it's time to make that connection happen.
 

What you see here are Reed switches. A Reed switch is an normally open (NO) or normally closed (NC) type of switch, (similar to how a light switch in your house works). And these Reed switches can be setup to be either NO or NC, but I'll be setting them so they are normally open for this project.

To activate a Reed switch to open or close, you use a magnet. Once the magnet gets close to the switch, it changes from its current position to the other. Example: if the switch is setup to be normally open and then you pass a magnet next to it, the switch will close. It's a very simple type of switch that I've used many times because they work really well. In fact, these get used in burglar alarm systems, like on windows and doors.

I'm going to use some 6-32 screws to mount these switches to the aluminum plate but my screws are too large to fit through the mounting slots. So I used a small square file to open the slots a small amount. If you look close you can see the bottom one has slightly larger slots than the top one. I did this on all three switches.
 

What I'm going to do is mount these Reed switches next to each block. I've drilled and tapped two holes next to each block (arrows). Once the blocks turn 180 degrees, a magnet inserted in the blocks will come close enough to activate the Reed switch.