Did you know that, apart from Russian collective numbers (двое, трое, четверо, etc.), we have the adverbs, based on the collective numbers (вдвоём, втроём, вчетвером, впятером, etc.)?
Collective numbers are used when we speak about groups of people or animals, and adverbs are used when we speak about an action, performed by this group.
Нас было трое в комнате. There were three of us in the room.
Мы втроём поехали в отпуск. The three of us went on holiday.