If your tank has 3 lines coming out of it, they should be as follows:

One line should bend up near the top of the tank. This is your pressure/vent line. It gets connected to the nipple on the muffler. Use one color line for this.

The other 2 lines should have pieces of tubing and clunks attached to them. Both of the clunks should be NEAR, BUT NOT TOUCHING THE BACK OF THE TANK. Take a second line of a different color than the pressure line and attach it to one of the brass tubes. Run this line to your needle valve on the engine.

The third line is for filling/draining the tank. Put the third piece of tubing, again, a different color, onto this brass tube. This line MUST have a SNUG FITTING plug for the end of the line.

So what you have now is, 3 different color fuel lines coming out of the tank. The line attached to the pressure/vent should be one color and goes to the muffler. The next line, which has a clunk on it, should be a second color and attach to the needle valve of your engine. The last line should also have a clunk on it and be a third color. This line must have a snug fitting plug in it (usually they are black). This line is for putting fuel into and removeing fuel from the tank. The plug MUST be inserted in the line for the engine to run properly. If the plug is not in the line, no pressure can build up in the tank. Also, remember to take off the muffler line when filling/draining the tank and put it back on afterwards.

Hope this answers your question about the third line.

You also need to check the height of your tank. If you have the throttle open and look inside the carb, you will see a brass tube sticking across the inside of the carb. This is the spray bar and should be even in height with the tube that runs from the tank to the needle valve on your engine (or reasonably close - within 1/4").

If that is incorrect, it can cause the excess fuel problem you have.