it’s charging us 220 bhats for the ATM
That’s a charge added by the ATM operator themselves, unfortunately there isn’t much you can do about it - maybe look for a different ATM that advertises free/cheaper withdrawals?
3%
That’s a charge added by Monzo when you go above 200£ worth of withdrawals in a rolling 30 day period; it can be avoided assuming you won’t need more than 200£ of cash.
the conversation rate is less than a bank
That’s surprising - Monzo gives you the MasterCard exchange rate with no markup or other nonsense - have you fallen for DCC by any chance? You should always decline DCC and ask to be charged in the local currency (DCC is there to prey on people’s fears of legacy bank’s foreign transaction fees by charging their card in the card’s native currency, but even the legacy bank’s foreign transaction fees usually work out cheaper than the awful deal DCC gives you).