I am going to put in a 10000 BTU, 3000 watt, 240 electric fireplace. I will be using about 15 to 20 feet of 12/3 wire with a 2 pole 20 amp breaker. Would that be a problem?
Thanks

Nope: 3000/240 * 1.25 < 20 or expressed another way
And 3000/240 < 0.8 * 20

The 1.25 is a rating factor for continuous loads. It's close.

1.25 is 1/0.8

I didn't perform a voltage drop calculation, but it should be fine.