Line from the muffler should go to the tank pipe that points up inside the tank. You'd pull the line off the muffler when filling to watch for outflow that signals a full tank.

The line from the carb to the tank pipe that goes to the clunk can be used for filling, but with three lines, isn't usually used at all.

The third line is plugged except to fill. Unplugged and used to deliver fuel into the tank, right. Then plugged. It goes to the pipe that points down in the tank. Truth is, down or straight or anywhere doesn't much matter since it lives it's life with a cork in it.