Oxygen sensor 3 would be the one that is located on or near the 2-into-1 cat converter.
For the rear main seal, the transmission or engine needs to be pulled out and so it is all labor cost. If it is not that bad of a leak, I would recommend just keeping track of the oil level in the car and try to find a friend or something who might be willing to do it for a lot less. Just a thought.
