Rub A Dub Dub is a nursery rhyme.

Rub a dub dub,

Three men in a tub,

And who do you think they be?

The butcher,

The baker,

The candlestick maker,

Turn them out, knaves all three!

The use of "be" instead of "are" may be an early modern English variant, an example of the subjunctive mood, or simply poetic license to achieve a rhyme.