Here’s a design for a balanced 6 belt to 3 belt reducer for Factorio 0.15.x.
This design is only slightly different from our 6 to 2 belt balancer. It’s used in very much the same way, and very useful for unloading trains. Both the 6 to 2 and 6 to 3 designs are balanced, so you’ll get an even load exiting each without any uneven backups on the input side.