Another Factorio tutorial, this time showing how to move 3 belts into 2 in a balanced fashion.
In the cases where you need to reduce 3 belts to 2, and you want all three feeder belts to be drawn from equally, use this design detailed in our most recent video. If all three belts are carrying two different items, segregated into left and right lanes, this balancer will not disturb their segregation.