In at least 50% of their songs three of them sing, why not three mics?
The harmonies sing into one mic. It makes it sound more like a connected unit, without giving a different colorization to each vocal part that seperate mics tend to do.
I assume that you are talking about the early live shows.
A couple of wild guesses:
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This was in the early days of sound reinforcement, and the drums and guitar amplification was all onstage. A third mike would let too much guitar and drums into the PA mix, meaning that they would never be able to balance the vocals with the instruments (This goes doubly so for television appearances–in a good studio, it’s easy to get a somewhat decent mix onto tape with just a couple of vocal mikes.)
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It just made for good showmanship to have two of the biggest heartthrobs in the world share a mike.
Maybe 'cause that’s how they were used to recording in the early days. George Martin only had a two-track machine for their first two albums. “I Want to Hold Your Hand” which is sort of the iconic Ed Sullivan shared-mike song, was their first one recorded on a 4-track machine. It would take some mixing down to get 2 mikes, 3 axes, and a drum kit to fit into only 4 tracks, let alone only 2. They used a four-track machine up until Sgt. Pepper. And then Sgt. Pepper was a pretty amazing feat of mixing a hell of a lot of sounds down into only 4 tracks.
The only three-mic concert that I can recall was the one on the rooftops.
By that time, they had upgraded to 8 tracks.
Also, the fact that Paul played left-handed made it easier to share a mic with John than if they were both righties (or both lefties).
Paul usually shared a mic with George.