if your burm is about 3'...then a small rat is in order. Go to a pet shop that sells rats for feeders, and just ask for a small rat.
It might not be a bad idea to check one of the rodent suppliers here online, and order a dozen for future use, because youa re gonna pay twice as much at a pet shop.
Mice are fine. For small hatchlings.
Switch your burm over to small rats. In a month or so, your burm might take two smalls.Try one for now,and let your snake tell you if a second one is in order. He'll be hunting around his enclosure in a couple of days if he is still hungry.If so, the next feeding use two smalls.I've had growth rates of as much as 8 inches per month. I've averaged 4".By 5ft, it's time for mediums. Some folks even use large rats at that size.
And please...get your self a book on burm care. In less than a year you will be dealing with a very strong, 7ft or more snake.
Take the time and learn.
A large hungry burm can be a dangerous animal.
Ask questions here, but go get yourself a book...perhaps even two different ones. And learn all you can.
Let us now how your snake is feeding and growing.
Bob