I'm just getting up to speed with the multi editing function in FCP.
I just learned how to sync up clips, and and found a few tutorials to get me started.
The 3rd camera is a 12 minute clip, which I needs to be synced up to the other 2 clips, which are much longer. This 12 minute clip starts while the show has already commenced, and before the show has ended.
I get how to sync up the first 2 clips using my in or out points, but can't figure out how to sync up this shorter clip. All I can think of is adding slugs to the heads and tail of the short clip so it's the same length as the other clips, then export the now new expanded clip to a self contained clip, so I can give it an in point.But I'm already seeing problems with this method, as I'll have no sync reference at the head or tail. Thanks.

Learn how to use the Aux Time Code setting on your clips or figure exactly how much of a slug you need to catch up the third clip and make a sequence out of it and the slug and make a QuickTime movie of the sequence and bring that into the multi clip or just edit the 13 minutes in manually after switching the 2 complete clips.

Or get the plug in 'plural eyes'. I use that plug in all the time for doing multicamera event shoots that I do by myself. I start recording on one camera, then run over to the second camera and start it recording, then run over to the third camera and start it recording. So the clips are not the same length, but plural eyes always lines them up just great for me. Of course, you have to remember, in your case, that it will look like nothing has happened until you get to the place the third camera starts, then the third camera view will just appear in the multitrack, ready to be switched to.

If you don't have THAT many footage, what I used to do is:

I made a timeline with the 2 or 3 videotracks on it; on top of each other. I putted them in such a way that they were perfectly in synch with each other.
I then put a in and out-point on my timeline, and exported the 3 videotracks seperately, with their own audio in the same format (which is needed for multi-cam editing!).
It's a bit of work, and you get a couple of files extra (although you can delete the old ones if they are in synch), but it works great.
If you have one clip that's only 12 minutes, and for instance, the other ones are 60 minutes, then you could give that video track only the same out point, but an in point which is much closer to it's beginning, so you have to export less.
Then just make sure when you go to multicam-editing, you pick : Synch using Out point.

If you use my method, and ignore the last comment, you can sync on In, Out-point OR timecode, they will all match. Of course, this method is not really usable if you have tons of footage. But if you have TONS of footage, it's better to just have a sync timecode while recording.