3 = A at which J J W W W G DuP T G C of his M

3 = Age at which... (see below):

James James
Wellington Wellington
Weatherby George Dupree
Took great care of his Mother
Though he was only three

It's from Winnie the Pooh, Now We Are Six, I think (maybe it's from The House at Pooh Corner?)

Edit: Yes. The poem is named "Disobedience," from Now We Are Six. The complete text can be seen at
http://www.umiacs.umd.edu/~ridge/local/disobedience.html.