I think 3 or 4 boys is 1 or 2 too many if you look at the first line of the chorus -

The one says aye and the other says no

seems to indicate that there are only 2 jolly boys after all! I suspect the folk process has had a hand in this and when the Spinners sang it I would assume 4 (like a four piece I know sing as well) while a trio would sing three. Idealy I suppose it should be done by a duo:-)

Cheers

DeG