How difficult is it to change the O2 sensors on a 2000 Mitsubishi Eclipse GS 3 ltr?

If I'm not mistaken...u have to take the valve cover off and it should be located right there

The GT and I didnt know a GS has a V6 option, but anyway they would be put in the same way, but you have (3) O2 Sensors! I don't know which you are replacing, But you have one on each exhaust manifold. The one on the front side (closest to the radiator) is almost in the middle of the manifold, has a couple wires and the wires lead to a square block connector, and is fairly easily removed with the sensor wrench. The other is a bit hidden. is is almost against the firewall on the left (Looking front to back on the car) protruding outward toward your left on the back manifold as you are currently looking at the car. That one will cost you a little blood to remove as quarters are a bit tight accessing it, you will need a swivel on your ratchet. That one also has a wire set that leads to a square block connector also. The third... underneath the car look past where the two exhaust pipes come together and you will see it protruding out of the exhaust pipe.

It is beyond me why you would need or even want to take the valve cover off, evidently he doesnt have a 3.0l, and still cant figure out why he wants you to take the valve cover off. Think he is lost :) But anyway that's where your (3) O2 Sensors are located at on the V6 3.0l on a GT and should be the same on your 3.0l.............