The punch line in that Limerick is everything but proper German (all for the sake of a rhyme on 'hindus'). A suboptimal shot at a translation is as follows: The build-up lines are easy:

At the banks of the river Indus
sat three meditating hindus.
And for several days
they are bother by the question:

and then the really bad German punchline:

"Is I it, are he's or am you's"

Wolfgang