What are your filters? Do you have DI?
You certainly have a sediment filter, and a carbon filter. The 3rd is either another carbon, or DI.
If you don't have DI, the water must go through the filters in this order :
Sediment, carbon, carbon, RO membrane
If you have DI :
Sediment, carbon, RO membrane, DI
On one side of the membrane plastic box (for lack of a better term), there is only 1 hose connector. Water enters there.
On the other side there are 2 connectors. The one in the center is where the good water exits. The one which is more on the side is reject water.
For the filter cartridges, if you look at the lid, you'll see that one connector goes to the side, and the other goes right in the middle of the lid. Water comes in on the side, and out in the middle.
Now all you have to do is connect the dots!
BTW, if you have DI, it's mostly useless for freshwater, but necessary for reefs.