If you plug the input between the injectors, and hole #2 you can run the skimmer with one pump.
If you do this the skimmer is no longer recirculating, which is fine, just less eficient.
After you plug the input between injectors and hole #2, you take the blueline pump, have it pull water from your sump, and the output from the pump goes to injectors, or hole # 1
Then the return from skimmer, #3, drains back to sump. It is best to have the skimmer elevated above sump so there is no back pressure on drain line. You want it to drain freely.