A triple room hotel in Thailand requires children to pay extra. Is it true?/You don't say.

Every hotel has different requirements. But the principle is this:

If you book a suite for a family, you don't need to add money, but the suite is expensive.

If you book a big bed room, or a twin bed room with children, many hotels have to add money because you have to add beds. It's no use squeezing with adults, foreigners do things according to the rules.