My computer is bad could I put a used one on my car
Sure, give it a try. It will either run or it won’t. You’ll either know what the problem is, or you won’t. Just swapping parts rarely fixes a problem.
I don’t think the engine will start, you will have an ID mismatch between the used PCM and the Immobilizer module.
There are seven different computers for your vehicle.
https://automotix.net/usedautoparts/2003-mitsubishi-eclipse-engine_computer-inventory.html
When I get a used computer locally, I have to provide the bar code of the old computer to acquire the correct one.
Tester