Reliably mix 3 ingredients for a factory

I took @Fambida and @Paul Z's suggestion to use long inserters, and came up with this:

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Should be jam-proof.


Consider making the cogs from iron where they are needed and inserting them directly from one assembler to another.

This way you only need 2 items on the belt, and you completely avoid the difficult issue of feeding your belt with the correct ratios of the 3 items.