Greensleeves arrangement?

I am attempting to write an arrangement of Greensleeves for a br trio, consisting of French Horn, Euphonium, and Tuba. Being the Euphonium player, I read b clef, but know enough treble to get by. I am having trouble with the transposition between the Horn (in F) and Euphonium and Tuba (in C). How do I get it to work?