The small line is the gas tank vent.  Typically it runs to a canister and ultimately vents to the intake system, often through additional hoses and one way valves. You didn't mention the year of your car. On the earlier cars there is a blue canister that bolts near the frame rail. The latter cars I don't know.

If you have three lines one goes to the fuel pump , one goes to the carbon canister , the last one which is a very small line goes to the fuel pump return line ( if you had a 2.3 ) for anti vapor lock , your carbon canister should be in the drivers side door , if it has not been removed, the line goes to the top of the fuel tank , (vent ) , the other line is the extra line on the fuel gauge sender ,