Which episode was the beginning of the end for south park?

This episode has yet to air.
The Simpson, on the other hand...

Which was the first 3-part episode? Whenever that was, that's when.

would had been kino

the 2016 election arc. It was just entirely bad

Make Love Not Warcraft

It’s still a great episode by all means but it definitely felt like a half hour long advertisement and marks a shift in tone for the series (second mention goes to Chef’s death episode also in Season 10).

After the chef leave

why did they do it bros?

Season 11 was where it started to get bad, especially with the second half which they put all their ideas into the Imagination Land episodes which honestly weren't great. Where I completely lost interest in the show was the post 08 election episode. It was just really shitty and underwhelming and I just gave up on it.

Whenever they started using dramatic music cues several times in every episode, as if to beat you over the head with "THIS IS THE CONFLICT, LAUGH NOW!"

Stanley's Cup should have been the finale