This post/thread on the SUF might help:
How CoH Works - Retail and Steam builds working together! - Steam Users' Forums
The three games all merge in to one (ie., I can double-click on "Company of Heroes" in Steam, but I have access to all "Opposing Fronts" and "Tales of Valor" content within the one game).
My understanding is that "Owners of Opposing Fronts will be able to play against owners of Company of Heroes and vice versa, although only using the armies from the game they own. Owners of both games will be able to play all four armies in multiplayer." - I haven't actually tried/confirmed this for myself, though.
(Later...)
My (limited) understanding of CoH is that Tales of Valor added the "Operations" to the multiplayer aspect of the game - that being the case, a quick check shows that right now there are far more "Classic" or "Skirmish" games on-line than "Operations" games.