I wish there was a way to split 2 copper this way and 1 this way, for example. There needs to be a way to prioritize certain things with splitters. Like my copper wire needs 2/3 of a belt of copper to run, but it can get very crowded, with 2 splitters here and more here. I just wish there was a way to make more specific requests from splitters.
You could get pretty close with circuit conditions. Only output on the 1/3 side if there is over X on the 2/3 side. I don't think any of the belts handle item counts divisible by 3 but again, you could get close.
Send it ALL down the way you want 1/3, and when it backs up, the rest will go down the other way. If there is no backpressure, then you probably need to produce more so you can send it down the other way from a different belt on your bus.
I've made a Ratio Splitter with circuits, not splitters, see if you can use it. You can set any proportion you like with it into any number of channels. Best use of it is leeching the main raw material lines. You can check it here: viewtopic.php?f=193&t=64740#p397076