Is it true that mainstream media?Democrats have been predicting President Trump's political demise since he was elected THREE YEARS AGO?

To some extent yes.  But what you've got to realize is that Trump is going to be in office until at least January 2021, regardless of what goes on.  The only way his "demise" would happen otherwise is if he died, resigned, or was impeached and removed from office. 

The predictions of Trump's demise weren't wrong because he hasn't screwed up badly.  They were wrong because media reporters overestimated how serious Republican politicians were.  They assumed that Republicans actually cared about things like good governance and the welfare of America and would buck Trump when he did things like, say, praise Neo-Nazis as "very fine people", side with the Russian dictator against America, or invite the Taliban to Camp David to accept our surrender in Afghanistan.  The problem wasn't that the media hated Trump, but that they had too much respect for Republicans.  They didn't realize that your average Republican politician is completely craven and has no regard for the law, the constitution, or American prestige and power. 

But the media is absolutely right that Trump has been a political disaster for Republicans.  They've lost badly under his rule.  2018 was a bloodbath like Republicans haven't seen since Watergate.  Think about that.  With the economy doing good and the nation more or less at peace, people delivered the GOP a loss unlike any since the party was linked to the greatest scandal in American political history.  The bloodletting actually began in 2017.  While Republicans managed to hold on to several House seats which were up for election, they lost a Senate race in Alabama of all places.  They also lost the Governorship in New Jersey and failed to take it back in Virginia, despite Democrats running an uninspiring candidate.  Republicans continued to lose badly in 2019.  They lost the legislature in Virginia, delivering Democrats unified control for the first time in decades.  They failed to unseat Louisiana Governor John Bel Edwards and they lost the Governorship of Kentucky.  The polls for 2020 show that Trump loses to basically every Democrat running, sometimes by large margins.  And you can't dismiss those polls as fake because they are backed up by the internal polls of the Trump campaign, which obviously wouldn't be faked in Democrats favor. 

I blame Nancy Pelosi for waiting this long. 

It's true that he's been behaving in ways inconsistent with our norms, laws, and constitution ever since he was elected, so yes, few sensible people thought it could go on for long.

Yes.  And, they haven't accomplished much else.  

The deep state and their MSM and Democrat flunkies decided that Trump committed a crime by winning the 2016 election. 

The  lamestream  media  is a arm of the democrats