I believe there is a 10,000 bonus for every 60,000 Marriott points transferred to United.
225,000 Amex to Marriott (with 50% bonus) = 225,000 Marriott Bonvoy points
60,000 Marriott = 30,000 United (60,000/3 + 10000 bonus)
225000 Marriott = 105,000 United (180,000/3 + 30,000 bonus + 45,000/3)...