3 objects parented to one path

I’m trying to move these objects down a conveyor belt. I can do that, but I want two objects to disappear at one point and the other to appear at that point. When I change the start frame or stop frame for one object, all three change to the same setting. All I can get is the three objects moving down the belt together. I’m still learning so maybe I should use keyframing instead? After I get it working I’ll put more of the same things on the belt. I’m thinking of using an IPO to animate the alpha to make them show or not show, but I’m still struggling with that. Also I would like the belt to stop periodically. Has anyone done something like this before? I could use a little guidance.

You could try using 3 separate paths going the same way, one for each object.

you can make they appear by keying their layers i think.