Amplifying Current from 0-20mA to 3A
Buy a VFD or other appropriate motor driver that can take a 0-10 V analog input.

You don't want to "amplify the current" if you can get away with it. For driving a motor, it's far more efficient to use PWM or similar. All you need to do is take that 0-10 V analog signal and use that to control the PWM duty cycle. This can be done with analog circuitry or with a microcontroller. You'll need to follow that up with a high current output stage, such as a power transistor or H bridge. You can probably buy a box off the shelf somewhere that does all that.