TMZ did some digging, and it turns out Ken Jennings, Brad Rutter and James Holzhauer have chosen to splurge (or not splurge) on unique causes -- and you'd be surprised at who's done what with their dough. BTW, this new 3-day tourney's got a million-dollar cash prize.

That said ... let's get into the numbers a bit.

Starting with Ken, the guy's racked up a grand total of about $3.5 million in "Jeopardy!" winnings over the years between his regular 74-game streak in '04 and other tournaments he's competed in since. Looks like KJ's an investor type -- he's got quite the portfolio.

Ken's got a crib with an estimated value of $2.1 million in Seattle. He's also pledged 10% of his OG $2.52 mil haul to the Mormon church and has announced a couple other donations. Other monies are tied up in stocks and bonds.

James seems a bit more charitable, at least publicly. He's pulled in around $2.7 mil from "Jeopardy!" and a good amount has gone to charities like Project 150, Communities in Schools of Nevada, Vegas-Clark County Library District Foundation and the Lustgarten Foundation.

The orgs support efforts range from preventing high school dropouts, helping homeless youth, pancreatic cancer research and, yes ... good old-fashioned reading and learning.

Now, for the biggest cash winner, Brad ... who's a little more low key with his purchases and donations. He's pulled in around $4.6 mil in "Jeopardy!" dough but it looks like he owns just one house (from what we could find) in the Hollywood Hills worth $1.6 mil.

He's donated some -- $100k to his home county in Lancaster, PA -- and appears to have been involved in funding public libraries there as well. Everything else ... bit of a mystery.