Forbes has also said he badgers them to inflate his rating, and they fault themselves for having given in to that over the years. If their estimate of 3b is very recent, then it might fall in the stage when they're trying to do a more accurate assessment, but if it's from a while ago, it might be seriously overstated.
Of course, he could just release his tax returns; then we could just know. The fact that he doesn't want to do that suggests that the actual value is lower than the lowest reported value (or he'd do it to boast).
In a deposition, he once famously said his wealth is whatever he
feels it is:
https://money.cnn.com/2011/04/21/news/companies/donald_trump/index.htm
Yeah, he just likes big-sounding numbers--bigly big.