http://www.infoplease.com/dictionary/brewers/we-three.html

CapriUni, your picture is missing a vital piece of the joke - that caption! If you took a Punch or New Yorker cartoon and removed the text, it would be equally meaningless and unfunny.

Basically, your updated picture is giving the setup but has no punchline. With the punchline (say, a caption of "three asses") I have to say it would still be a rather obscure joke. I suspect the joke would also have been obscure to anyone in Shakespeare's time who didn't know about that pub sign or the artist who painted it. As an art reference, it's as obscure to us as a reference to a shark in your roof might be in 200 years (and that might even be obscure to people today who aren't in the UK).

Graham.