I'm afraid you really don't have time for Mrauk U. If you fly to Yangon on the 27th, you would start for Sittwe on the 28th, and the best you could hope for would be to get to Mrauk U late evening on the 28th. You could have the 29th and 30th in Mrauk U, leaving before sunrise on on the 31st to go back to Yangon. This would require that you hire a private boat both ways ($$). And by flying back to Yangon on the 27th, you are shorting your time in Bagan. Save Mrauk U for your next trip. I don't see any way you can get three days at Mrauk U. In addition to the flights, the boat trip is a good 5 hours each way.