My bad & you're right... it was Phil Hill & AJ Foyt that won at LeMans... as well as a few others.
I am also wrong about a NASCAR championship... The feat was as the only driver to have won a race in the NASCAR Sprint Cup Series, Formula One, and an Indianapolis 500.
USAC used to run a stock car series as a competitor to NASCAR, and if I am not mistaken, Mario won some of those races and maybe the Championship ... so that could have been the stock championship to which he is credited. I believe Bobby Unser also won a bunch of those races. I am too lazy to google it.
When Dan Gurney and AJ Foyt won Le Mans in the Lola leftover known as the Ford GT40, Dan Gurney became the first man to spray champagne on the podium ... a tradition that survives to this day.
I have met Phil Hill a couple of times. He is a great raconteur of days gone by and all-around gentleman. I could listen to him speak about il Commandatore, races and racers long forgotten until the cows come home. He raced at a VERY exciting time in Ferrari history when being a factory driver meant you got the best cars for the Mille Miglia, Le Mans and F1. He is still the only American-born racer to have won the World Championship. As you can probably tell Phil Hill is a personal hero of mine.
PS Juan Pablo Montoya has duplicated Super Mario's feat of winning Indy, a NASCAR race and a F1 race.