Force = (mass) x (acceleration)
Force = (18 kg) x (3 m/s²) = 54 newtons
As long as you continue pushing the cart with 54 newtons of force,
it will accelerate at 3 m/s².
At the instant you release it, or keep your hands on it but stop pushing,
it will stop accelerating. It'll continue forward at the speed it had when
the 54 newtons of force stopped.