All I gather from your post is:
1. There are 6 of you - 2 supposed to arrive first and 4 the next day, for which you have 3 rooms
2. But 4 arrived on the first day instead of 2

Here is where you are not clear - did you book all 3 rooms for 4 nights? Or was it only 1 room for the first night and 3 rooms for the next 3 nights?

Was it the latter? Where did guests #3 and #4 sleep on the first night?