3 way valve?

I'm calling it a 3 way valve because I don't know what else to call it. I'm looking for a valve that if installed, say in a toilet, would shut off or turn down the toilet fill while someone is using water elsewhere, say taking a shower. This would prevent someone from being scalded in the shower is someone else flushed the toilet.

Shower valve

I do not know of any such valve...
The correct fix for your problem would be to install pressure ballanced shower faucets. Or to install a pressure ballancing valve on the supply lines and a thermostatic mixing valve on the hot supply line to the existing shower faucets.

valve

There is no such item available. You could make a system yourself using electric valves and sensors but it would cost a lot more than just replacing the valves with pressure balanced ones.

It would cost more than a new HOUSE!