When entering random dates for a stay the top of the Search page indicates...
ABOUT THIS HOTEL
Maximum 7 accommodation per booking
Maximum 4 people per accommodation
Maximum 2 adult(s) per accommodation
Maximum 1 child(ren) per accommodation
Children : From 12 years old, children are considered as adults
While it states max 4 people per accommodation, 2A + 1C = 3 people, which would seem to link in with the OP's issue.
Bed arrangement is also 2-single or 1-queen/king, which would be ok with 2A+1C but not 3A.