I realize this saying the post catalytic converter sensor is bad but my question is just replace it or figure all three are not long for this world and bite the bullet?

I could be convinced either way.

 

Two ways to look at it:
1. Pay me know or pay me later
2. Don't fix it if it ain't broke.

I would go with option #2.

 

Second that.

 

p.s.
buy Denso.