The third line that runs up with the fuel lines, what exactly is it used for, and is it important? For some reason it is plugged and has been since I got it a few years ago. Any idea why it would have been done like this?

Looks like this. You can see at the top of the picture the other side where it is suppose to connect to has a cap over it as well.

 

When the fuel tank vents, the hydrocarbon vapor is absorbed in the carbon canister in the driver's side rear fender. While the engine is operating, the EVAP purge solenoid opens, and allows vacuum from the throttle body to pull air through the canister, pulling the hydrocarbons into the engine to be burned. Reduces HC emissions from the tank, probably contributes a tiny amount to fuel mileage.

If the system doesn't work, the canister saturates, and the back of the car may stink of fuel when the tank vents.

Can't see your picture, because I'm at work, and the system blocks that site.

 

Thanks for the info.

any idea why someone would have disconnected it?

 

I disconnected my complete EVAP system, but its a "track" car, and doesn't go through emissions testing.

 

Some people like to disconnect things they don't understand. It's a pure guess why it was disconnected.