Let's imagine the next situation:
Let us assume that we have no way to put next to furnace an inserter.
Well. Now, we insert a something into chest, and what's going on after? Upper inserter will take, and don't stopping while conveyor not full. Solution? Programmable inserter. To solve this problem, we will do following steps:
1. Put chest to center (where 4 inserters)
2. Replace next to chest 3 simple inserters to programmable.
3. Link inserters with chest in center.
4. Make a condition:
if (linked.getCountOf("%insert name of stuff here%") >= 3) { linked.take("Coal"); }
This is schematic, in real game it can be replaced on GUI, for example.
Well. That's all.
I don't know about this.