Here is our Factorio tutorial showing how to build a 3 to 1 belt balancer that reduces the number of belts used while drawing an even amount from each incoming belt.
This 3 to 1 belt balancer is particularly useful at train depots, as two of them are sufficient to provide egress for all 6 inserters capable of unloading from one side of a single train car. If you are carrying two items on each belt, each segregated into their own lane, this belt balancer will keep those items separate in the same lanes after they’re combined into 1.