Let's say Fernando Alonso wins the indy500, and then next year wins the 24 hours in a GT car, would that mean he has won the triple crown? Or does he have to do it in the LMP1 cars?
I think it has to be an overall win. Mario Andretti (1st WDC, 1st Indy 500, 2nd Le Mans) was close to this in 1995. He finished second in a CourageC34 car. The same can be said to Jacques Villeneuve (1st WDC, 1st Indy 500, 2nd Le Mans), finishing second in 2008 with Peugeot at Le Mans. Juan Pablo Montoya (Monaco GP and Indy 500) could win the Triple Crown also.
Extra note: Fernando has said he is only interested in an overall win, not a class win. So he won't be racing a GT car. Thus the LMP1 is his only chance.