There's two bolts that hold the coil on to the plate. No need to take the whole plate off. I haven't seen many coils go bad. Usually it's a bad connection or a bad CD module. In your case, one section of the CD module that drives #3 coil is not used. You can wire it up as a test.
Be very careful when messing with the wiring on the Prestolite ignition system Very easy to get the wires crossed up. The wire colors on the CD modules are the same for both of them. The wires are position dependent. That is it depends on where you connect them on the terminal block that determines which coil they are connected to. Take pictures and write down where the wires go before disconnecting any of the wires.