Here's a handy little map.
A -- B -- C -- E
Those are all planets you control, and phase lanes. Extractors in each of those have a refinery quota of 3, meaning they can only serve 3 refineries at once. In addition, a refinery has a range of 1 jump.
So if you build a refinery at A, it will hit A and B. One at C hits B, C, and E. One at D hits B and D.
One at B hits A, B, C, and D. In this case you want to build 3 refineries at B, and 0 at A, C, and D. You'll cap the extractors at all four planets and come out ahead in income. E is left out, but if it was connected to another planet (F), you could build refineries there and get both that way. I wouldn't build at E on this map, because C is already capped and you will only get more from E, which isn't worth the refinery cost.
Hope that helps.