What big 3? I can count at least 8 brands off the top of my head which sell a variety of vehicles. Tesla started relatively recently, and I have seen a few Rivian around. Kia and Hyundai made large gains in the last 30 years.
https://www.whichcar.com.au/car-advice/car-manufacturer-bran...
We have numerous Korean, Japanese, German, and American vehicles on the road with many varieties of sizes and performance.
And obviously you will need to have deep pockets to enter the car business, why wouldn’t you for such a complex machine? That does not mean there is excessive consolidation in the market.
"The big 3" is a phrase referring to the US automakers; Ford, GM, and Stellantis (formerly Fiat-Chrysler).
On a global scale you also have giants like Toyota, Volkswagen Group, Hyundai Group, Renault–Nissan Alliance, and Geely.
These companies make most of the brands you've heard of.
Obviously the barrier to entry to manufacture a hugely complicated machine will be high, but the data indicates it is not impossibly high.
When Toyota sells 10 million cars a year it's not news, they have basically reached peak growth. Tesla's valuation is higher because they keep selling more cars and still have room for growth (and because of Elon Musk himself).