O2 Sensors

How many o2 sensors are there in a 99 Seville STS, and if anyone has the info on their location that would be great too. I need to replace one of them and OBD2 is reading the 3rd sensor, dont know what it means or where it's located, maybe before the converter?

Bank what, sensor what? Usually there is one in each engine pipe from the manifold and one behind the cat, some have a sensor just in front of the cat as well as the other 2 in the pipes just off the exhaust manifolds. The code pinpoints which one.

It reads bank 1 sensor 3. I think it might be post converter? Maybe?

Can you post your codes? Bank 1 sensor 3 is located just past the converter.
Note: If your code is P0420 its not the sensor, its the converter.