A monopoly only exists because the government itself creates it. For example, USPS. The reason we have to subsidize the wages of mail service, have to fund their pensions, have to keep giving money to rich millionaires who do very little, is because the law is, no one can compete with USPS.
Equally, in the 1940s, the FCC created 48 white spaces in the radio spectrum specifically for broadcast TV. Yet how many channels did we have? Three. NBC, CBS, and ABC. Fox came later. The UPN network tried to startup in the 1940s, but the FCC shut them out of the market, protecting the tri-opoly of NBC, CBS, and ABC. It was government that created that protection of the big three.
This has been seen numerous times in US history. Another example would be the auto manufacturing sector. In the largely unregulated 1960s and prior, there were dozens of independent car companies. After the regulation of the 1970s, and 80s, by 1990, there were just three big manufactures. The rest were forced to sell off by government regulations.
In a true free-market capitalist system, anyone can engage the market and compete. A few people could start a new car company and compete with the big three. Instead, government regulations make that impossible, creating the very monopolies you claim to be against.
Your post is full of inaccurate information.
There were far more than 3 TV channels. Not networks, channels. But there were no communications satellites. Each station built a tower and their signal only reached so far. 40 spaces were needed on the spectrum were needed because each station's signal had a relatively limited range and there could be no overlapping signals.
Where we lived, we had three stations with rabbit ears but those with a rooftop antenna could get 8 or 9.
There was no prohibition against small independent stations but they didn't have the resources to produce programming. The networks were formed by radio networks who did have the money to produce shows and many of the early TV shows were radio programs, which the networks already had rights to and talent under contract, that they transferred to TV. No independent station could compete with those resources.
It wasn't the government that prevented more stations from going to air, it was the economics of the industry itself.
As for car manufacturers, there have been numerous car companies over the years and again, government had nothing to do with their success or failure. Some flourished for a time but none achieved the kind of commercial success or size needed to last beyond their founders' lifetimes.
The Tucker appears to have possibly had a chance, but the Big 3 refused at allow their dealers to sell the cars, and they actively worked to undersell and put the Tucker dealers out of business.
John Delorian talked about the near impossibility of starting a new car company. It's difficult to raise capital because the odds of success are so small. You have to get the car to market quickly, and it can't have technical issues. You only have one chance to make a good impression with the public.
Then there's the dealer network problem, advertising and promotion. Even the big car makers have problems with introducing new lines.
Tucker, Delorian, Bricklin, all tried and failed. So did many others. Not because of government regulation but because of the economics of the industry.