question

Whenever your conveyor routing logic needs to be based on something more involved than simply reading a label of the item at the DP, then you typically will want to consider making a process flow to define where items should go. In your model, the routing logic may seem simple at first: "send to the first conveyor until it fills up, then redirect items to the next conveyor". But there's other things to consider, such as when is it ok to start sending items back to conveyor 1. You probably don't want to send items back to conveyor 1 until conveyor 2 fills up and begins releasing. Or maybe you would prefer to send to conveyor 3 next, and only when conveyor 3 fills up, start sending to conveyor 1 again. You will also need to concern yourself with what to do when all conveyors have filled up and/or are releasing simultaneously. In that case, you may want to divert items to another circulation conveyor, or stop the whole conveyor belt.

If the desire is to send a slug of items to conveyor 1, then conveyor 2, then conveyor 3 in a round robin fashion, then the following Destination in your OnArrival trigger code at the main conveyor's DP8 may do what you want (where 15 is the slug size):

current.outObjects[(trunc(current.labels.assert("roundRobinVal", 0).value++ / 15) % current.outObjects.length) + 1]

Here's what it will look like: