That 3rd upstream is used only on the 96,97 5.0Ls. Tunercat only shows it in the DTC list for those years. It is B1S2. The 98-99 5.0L just use B1S1, B1S2 and B1S3. B1S3 is after cat. Those are the only ones in the DTC list for those years. If you are doing a 0411 swap you will be using a later year OS so you will only need B1S1 and B2S1 O2s.