1st- Your body will keep going at the same speed in the same direction unless some other force acts on you to change that speed or direction. When the coaster speeds up, the seat in the cart pushes you forward, accelerating your motion. When the cart slows down, your body naturally wants to keep going at its original speed. The harness in front of you accelerates your body backward, slowing you down.
2nd- object depends on the object’s mass and magnitude of the force acting upon it (F=ma). You feel this second law when you start going down the hills. The coaster cars and your body have mass. The gravity provides acceleration. That causes force. The rider feels the force as it moves the cars along the coaster track. The track directs the force and the cars. The mass of your body and the coaster cars is constant. The amount of force changes due to changes in acceleration. The changes in acceleration are due mostly to friction.
3rd- As you push down on the seat, the seat pushes back at you. This is third law.