"Rub-a-Dub-Dub" is a traditional Nursery Rhyme for children. It is often sung by parents to their children at bath-time, because they are sitting in a tub (bath) and the parents need to rub clean. Turning bath-time into a game always helps if children are a little reluctant. Note that there are similarities among the the melodies for "Rub-a-Dub-Dub", Ride a Cock Horse and Little Jack Horner. PDF sheet music, Midi and Mp3 files for this melody can be downloaded using the links in the left-hand menu.
The image below is the first page of the sheet music, and here you can download the full sheet music for Rub-a-Dub-Dub for piano in PDF format.