If you stay in a "Concierge Level" room or "Club Level" room (or something along those lines) and you have three adults officially registered to the room, everyone in the room would legitimately have lounge access. The room description on Marriott.com will indicate that lounge access is a feature of the room. It has nothing to do with Elite status.

However, many of us on this forum get lounge access at most full-service properties because of our Marriott Bonvoy Platinum Elite (or higher) status, regardless of room category. In that case, the published benefit is for the Elite member plus one guest. In such cases, the hotel can charge for a third adult or deny access to a third adult.