This Factorio tutorial shows the design for a 3 belt to 6 belt splitter with balanced loads.
This design is very useful for train depots. A train car can only be accessed by 6 inserters per side, so you often need to split your items into 6 different belts to minimize the amount of time your trains wait at the depot. This design will take an even or uneven load from 3 belts and evenly distribute the items onto 6 belts.