Why is only 1 of 3 ignition coils not working. I tested it with a 12v

Regardless of engine (3.1L or 3.8L) coil failures are not uncommon on these cars given how old they’re getting. If you want to test the coils in a more direct fashion, here’s a good video I found for a 3.8L car which should have a similar coil setup to your vehicle. The only difference for the 3.1L and 3.8L is the placement of the coils on the motor, I think they may even use the same coil part numbers (but definitely check for yourself). If all three of your coils are sparking, then you may have an issue with plug wires.

If you have a coil that isn’t sparking, the cheapest fix may be to visit a junkyard / u-pull and grab a coil pack from a car with the same engine, figure out which coils work on it via the method above, and then swap the good ones (or whole assembly) in.