15 amp draw on air compressor on start up runs at 8 amps on trips breaker while there is pressure in tank do I need to up my service from 15a to 20 a or do I have to do both 12/3wire or stay with 14/3 wire

Generally we use a 20A breaker on #12 wire and a 15A breaker on #14 wire.

In your case I would use #12 on a 20A breaker.
You should not need 12/3 if this is dedicated to the compressor. 12/2 is typical.

Is it 120v or 240v?

Stan asks a good question that mede me wonder too. You comment about " stay with a 14/3". What is voltage of compressor? Unless your compressor has a startup compression release, it draws a lot of current to get going.

The amount of air in the tank should not affect the startup of the compressor if the unloader is working properly. The unloader releives the pressure on the pump so it doesn't start under a load.