Get your slimy, webbed phalanges off ma boots!
Can a bodyguard be added to a stall with 3 worker count already present? The rules say "A stall is considered full when the worker count plus overflow from adjacent tiles equals for or more." The "or more" would seem to imply so, but it isn't overtly stated as far as I can see.
You're 100% correct. Playing a bodyguard to a stall, when that stall's worker count + overflow is already at 3, is completely legal, and the only way for the worker count + overflow to ever reach more than 4.