after it comes out of the slicer. The sauce is tangy. As KC pointed out, it's somewhat similar to Arby's sauce.
I had a sister who lived in Boston for a while, so I've had a 3 way a couple of times when we and visited her. I don't like mayo, so I always ordered with no mayo. I guess it's not a true 3 way, but it's a very good roast beef sandwich regardless.