Pretty simple scenario, multigrouping with 3 cameras (5D), transcoded to Prores HQ. I am grouping the 3 cams with external audio, and doing so with INPOINTS. Final Cut is not taking into account the In Point Marked on my audio master clip, and instead is just adding it into the group from the head of the whole clip!
There might be a better way than this, but you could drop the A cam into a new sequence video only with the external audio, sync and link them, then drag that new clip from sequence into a bin to create the new master A cam clip. Then multiclip your new A cam with B & C. When you're editing you cut "video only" and leave the audio set to cam A.
Not as easy as automatic but not too bad if you don't have many clips.