It starts with the assumption left, right, and centrist sources all have biases, but those biases cancel out. That the "middle" of those things is meaningfully better.
I see no evidence that that's the case. All popular news sources these days seem to be biased towards corporate profit.
"Left" and "right" both exist relative to each other more than they exist relative to the truth.
Reading several news sources to find the truth begs the question of if modern news sources, regardless of political bent, are meaningful sources of factual information vs sources of advertisements and outrage.
For the sake of presenting a counter argument, I think that keeping up with the constant inflow of news doesn't get you meaningfully closer to the truth compared to reading well-regarded books on recent history around the world, and using that as a lens to understand the present. Well-regarded non-fiction books are generally better researched than stories on "this senator just had a haircut", and the recent past shapes the present significantly, not to mention repeats itself shockingly often.
By reading left and right sources, you're assuming you can find similar stories on similar matters, when the most important matters are not "the president tweeted X" (which both sources will report on), but rather information about the structure of cities, the human condition, and changes thereof ("a postmortem on this transit project shows that we, as a society, should think in this way"), which usually no news agency will report on. It's just too dry, unless the researcher dresses it up and makes it flashy.