I would imagine that right now there are times when the existing couple does something on their own, and everyone understands and is fine with that. Adding another person is going to need some of that same "just us" time. Are they adding this person as a triad, or as a Vee? I only ask because a triad will probably mean even more "just them" time (A+B, B+C, A+C, A+B+C) but except for the times when it's "just the three of them" one of the three may still be available to hang out with you as the others have their "alone time" as a couple.
I would say it's certainly fair to express to them that you're worried about being left out or feeling like a "fourth wheel", as it were. They aren't being given a chance to reassure you or address your concerns unless you bring it up. But try not to rush them for an answer or solution- they probably have a LOT they're trying to figure out right now.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack