(You booked 2 room but want to cancel 1 room ? and the other room want to live 3 people, right?)
In case I have told you that for "If you change or cancel the room and the website charge money from you or not refund your money" (If you booked with Expedia/Hotel.com on Promotion month)
But if you can change or cancel without charging from website , you can cancel then book us directly with 750 baht +200 baht .
the price on website now about 800 Baht if you had get lower price I think may be you booked the room on Promotion month