The DJIA in 1975 vs DJIA now is about 7x. If you go with an average of 6-7% interest per year it would be about 20x or $280 million. Either way, whether you go straight by DJIA or by a guestimated interest it is far less than $3 billion.Might need to run these numbers again chief. I don't think you'd only 7x your money over the course of 45 years in the market.
To be fair, I just looked up the annualized interest on an investment from Jan 1975 to Aug 2019. The annualized rate was 8.5%. So, given that, if Trump had done nothing but dump $14 million into the stock market and let it sit, he would have $500 million, still an order of magnitude less than $3 billion.
My point remains the correct, he had to have made a few good business decisions along the way.