Having them watch the motion of their lab partner as that partner pulls on them with a constant force reinforces the notion that acceleration is the continual increasing of velocity. Hopefully mass and angle of the track, which alters the force. That the steeper the track the faster the car went down the incline and that if the track height was maintained the smaller cars went faster. Have students make a rough sketch of a force versus acceleration graph would look like and a sketch of what a force versus mass graph would look like. Your body perceives accelerating forces but not uniform motion.
Students have a great deal of difficulty with predicting the correct reading on two attached scales.
It is harder with three. Be certain that the have the experiences to develop the notion of equal and opposite forces with the scales. Approximately the middle of the distance between them. Approximately the middle. Same place as before.