I doubt the company was divided 3 ways evenly, as that almost never happens.
As I said, Cuban, like nearly every other billionaire in the country, owned shares of the company he and his partners founded. The value of the company isn't the value of Cuban, just like the value of Microsoft isn't the value of Bill Gates+Paul Allen-it's much higher. As Microsoft had to go public to finance their massive growth many years ago.
In other words, he and his friends owned 100% of a much smaller, up-and-coming company. When they felt they needed a large amount of capital to grow, they had a law firm or other organization that specializes in IPO's (initial public offering) underwrite his company. That entity determines the market value of the company and divides that value among shares of stock, most likely with Cuban remaining the largest shareholder-possibly around 23% (just going off of the $5.7bn figure; 1.3/5.7).
Or maybe he and the other broadcast.com shareholders were given a combination of cash and Yahoo stock that equaled $5.7bn, which would be much less now that dot.com stocks are worth a fraction of what they were then.