The intimidating Mr. Harris hasn't left much of a historical trail. He isn't mentioned in either Joe Barbera's or Bill Hanna's memoirs. His appearances in the cartoon diary may be all that's left of him. He appears to have been a production bean counter and Quimby's bad cop on the floor of the studio.
I don't recall ever seeing an animation credit for Tom MacDonald on a Tom and Jerry cartoon. He might have been an assistant at MGM. He did get an animator's credit at UPA years later, working under Pete Burness' direction on some Mr. Magoo cartoons.
|Rough animation by Tom MacDonald for a UPA Mr. Magoo short.|
A Magoo short with some Tom MacDonald animation.
The cabinet can also be seen in use in this video.
Who the third vanishing animator is has been lost to binding of the original folio of Spence's diary.
*This blog also says that Bud Crabbe was an animator at Disney and Walter Lantz.