OK, I’m not too sure of this (besides, the other two are unofficial, anyway), but here I go.
First Triple Crown: 24 Hours of Le Mans, Rolex 24 at Daytona, 12 Hours of Sebring.
Second Triple Crown: 12 Hours of Bathurst, 24 Hours of Nurburgring, Total 24 Hours of Spa.
Third Triple Crown: Petit Le Mans, ?, ? (your call on that. Maybe 1000-km races?)
There’s also going to be a Junior Big Four: idlers 12-hour, 12 Hours at The Point, Charge of The Headlight Brigade, 25 Hours of Thunderhill