What will happen if I connected 3amps adapter to motor that's need only 2.1amps are that will burning the motor?
Assuming the voltage of your power supply is equal to the voltage required by your motor, then the supply will only output the amperage needed by the motor. If the motor needs 2.1 amps then that is what the supply will deliver. In general, power supplies output the current demanded by the load and not their maximum current rating.  It is important to match the voltage ratings, however, or the load could be damaged.