I have a Fleetland Air Compressor.  It has a 3/4 HP Motor on it now,
but I would like to change it to a 3 HP Motor.  Can I do this without
messing up the Air Compressor?

Unless there is something special about the Fleetland air compressor,
it depends on the 3 hp motor. If it runs about the same speed as the
3/4 HP motor, the compressor will scarsely notice the change, and it
won't make significantly more or higher pressure air. If the 3 HP
motor is designed to run twice as fast, The compressor will make more
air and higher pressure briefly, but it will mess up the compressor. A
one HP motor that runs 10% or 20% faster may give you a bit more air
and/or a bit more pressure, without damage to the compressor.  Neil

I will trade you a 1 HP motor for your 3 HP motor.