As you surely know, the great French artist Jacques-Louis David painted the very famous Death Of Socrates, along with many other brilliant neo-classical works.
What is less well known is that at one point he thought about painting Diotima Instructing Socrates. He made a sketch, which is now held by the National Gallery of Art in Washington. Apparently the sketch isn't on display, but here it is, linked from their web site:
So that's two of my three main characters.
Thanks to Jason Rehmus, who is @longstride on twitter, for tracking this down.