Can a hotel in Thailand book a double room for three people?

Generally speaking, only two adults are allowed to stay in a room. If it's three adults, you need to pay for an extra bed or book a triple room directly.

If you want to save money, you can just say that two adults will stay when you book a room, and the other adult will sneak in and see if you can try your luck to avoid the cost. If you are caught, you have to make up, and you can only make up on the spot. I suggest you don't do this, so as not to be embarrassed then;