This is a pretty standard disclaimer on the ADR. I've even heard people getting that note for groups of 1 and 2. I would not worry about it - in all of my trips, our group of 3 has never been split up.
(Note: the only place where I'm not sure how good a three works is the sci-fi if you want to eat in one of the cars. Those benches are really designed for only two adults, so someone ends up alone.)