Well, post-millennialism posits that Jesus will return
after a 1000-year "golden age of Christian prosperity and dominance," after Christians (not Christ Himself) have established the kingdom on this earth.
This is directly opposite the pre-millennial view, which holds to Jesus returning
before His 1000-year reign on earth.
Post-millennialism and nunc-millennialism are more similar to each other in that Christ will return at the close of the millennium (which is not literally 1000 years but a "fullness of God's time," but are opposite each other in that post-millennialism sees times and society getting better and better through this period while nunc-millennialism sees things devolving over that same period.
Even in saying all this, there are... nuances to all these views within the various camps themselves...
I'm sure that clears it all up...
It is quite amazing how such widely diverging views can coexist.
Grace and peace to you.