When you refuel through that third line, the overflow will come out of the muffler line (exhaust) when the tank is full. You can either disconnect that muffler line during refueling and watch for the fuel to squirt out when the tank is full or......if the end of the muffler itself is pointing DOWN and is outside the cowl, watch for it to come out there. IF YOUR ENGINE IS MOUNTED UPRIGHT AND THE MUFFLER IS LEVEL OR EVEN POINTING UPWARD, DISCONNECT THAT MUFFLER LINE. Failure to do so will fill your muffler with fuel and flood your engine.

I hope this explains it. It's actually easier to do than to explain. Again, you don't want to flood your muffler/engine so do whatever is necessary to avoid that. If your engine is mounted sideways or inverted just fill through the third line and the fuel will automatically come out through the tip of the muffler.

In any event, it's always good to firmly grip the prop and turn it over by hand once or twice before hitting it with the starter. This way you can be sure you don't have 'vapor lock' which you will feel as extreme resistance to turning over the prop. This could damage the engine.