One vessel used for heating strike water, doing a full volume mash and for boiling. Now that I've switched to such a simple brew day I'm not sure why I would ever go back. BIAB rocks!
I said 3 (boil kettle, mash tun, fermenter) - I heat my sparge water in the boil kettle & then transfer it to my (stainless) fermenter temporarily so that I can drain my mashtun into the boil kettle (if that makes sense).