What's amazing about so much of the early Beatles recordings is how much was recorded live off the floor, including vocals. With a two-track tape recording system, this was almost a necessity (listen to the punch-in edit of a harmonic solo in "Little Child" for an example of how awkward edits resulted from this limitation). Advancing to four-track made overdubs a little easier and so when it came time to record "Yes It Is" with three-part harmony, the Beatles took the obvious route, recorded the backing track first and overdubbed the vocals later. This became standing operating practice on Beatles recordings after 1966, as instrumentation became more and more complex. Here, it was just beginning: George was occupied during the recording of the instrumental track with the volume pedal on his guitar, which he had first used in the studio earlier that day, overdubbing onto his song "I Need You."
What has always impressed me with Beatles recordings is the ability of John, Paul, and George in the studio to give excellent vocal performances on cue time and again. Listen as they go from an (intentionally) poor guide vocal from John to superb three-part harmony in a matter of hours: the session for "Yes It Is" lasted from 5:00 to 10:00 pm.
Here are takes 1 through 9...
...and the final version as it sounds on the master tape (take 14):