3 burnable fuels in a tower. You decide the priority order to use them.  If the first one runs out, the second takes over.  And likewise with number three.  Will automatically revert back to a higher priority when available.  If your priorities change, just alter the tower filters. No need to mess with the splitters. The new arrangement will flow, as room on the belts allow.

Tip: If you don't have level 3 belts, lay it down, It will show as white. Do a sweep downgrade while it's white, to downgrade the belts.  Your bots will take over. As long as you have the splitter tech, you can use it.