On a side note, I'm curious to whether you could expand on this... I'm fairly young and don't understand which 3 channels you're referring to. On-air? On the television?
The major broadcast news channels on TV were ABC, NBC and CBS. If you watched any of them, you'd get roughly the same news. They were all sponsored by roughly the same corporations.
There's a video of Bernie Sanders in 2003, talking to some high school students about politics. (How does he not look younger 14 years ago? Was he born old? At least he's not aging at all. :p)
He was talking about how the news media has a slanted view of the news. It's the news that the major corporations want to tell you. One of the main channels was owned by GE at the time. He was talking about how GE only wanted people to hear its version of events. At that time, he told the students to watch Fox News once in a while to get a different perspective.
Today, if you watch CBS this morning, CNN, MSNBC and Fox News on the same day, you could think you're living in different countries.
A couple weeks ago, Fox News was concentrating on a story of a 14 year old girl who claimed she was raped in the school bathroom by a 17 year old man and an 18 year old man who were illegal immigrants and going to school with her. Fox News reported that the 18 year old had a criminal record while CNN claims he does not.
None of the other news channels aired that story that I saw. CNN has a print story about it with a very different narrative than Fox News. The story has morphed into the girl allegedly giving consent.
Different media outlets have widely differing views on what they want to stress in the news.
Polarization is caused by people watching different narratives with different agendas.
As a side note, comedy is the same. I was watching an interview with 2 of my favorite media people, John Dickerson and Stephen Colbert. When asked what news they read daily, Colbert mentioned the Reddit front page, HuffPo, NY Times, CNN (all of Trump's fake news sources) and a slew of liberal media. Apparently, none of his jokes come from the other side.