3V Air Pump loses force when drived by a TIP120 transistor
The TIP120 is a Darlington transistor, which implies that it has very high current gain, but it also has a high \$V_{CE}\$ when in saturation. The OnSemi datasheet specifies a \$V_{CESAT}\$ of 2.0V when passing 3A, so most of the voltage available from your 3V source is appearing across the transistor and less is available to drive the motor.

You would be better off using a regular NPN transistor with a low saturation voltage, or an NMOS transistor with a low \$R_{DS}\$.