Hi 450hp67SS,just to confuse matters further. Some models of GM vehicles pick up "power" from the Starter terminal for the loom as in my 64 Chev convert. Is this your third wire ?.This wire SHOULD have a LARGE loop terminal,like the battery cable. As other posts have indicated, purple or red should be from starter switch to solenoid. Yellow usually is the resistance wire to coil (9V) and one other wire(Blue,Red,Green I've seen many different colors here) from solenoid to coil to provide full 12V to coil on start.
If your "third" wire is any of these colors you should be able to solve it. If not,I suggest hooking up a 12V light to the offending wire and have a look at what it's doing when you operate your ignition switch. I assume you don't have a multi-meter ?. When you start on Auto wiring,you really NEED a Multi-meter,they will save you a lot of pain.
I'm backing that the "third" wire is the 12V wire to the coil. Just connect it up to the Start Terminal. Hope this helps you out......Barry