Currently I think only Alonso really has a chance to get the Triple Crown.
Imo the guys with the best chance after Alonso are actually Räikkönen and Hülkenberg.
Hülkenberg already has the Le Mans win. If Renault becomes competitive and Hülkenberg manages to win Monaco, he might go for the triple crown.
Räikkönen has the Monaco win, but is nearing the end of his F1 career. I think he might just decide to take a shot at Le Mans and IndyCar. He's driven NASCAR before, so he's not new to oval racing.
Other drivers who've won a leg of the triple crown, but with less chance imo:
Montoya has 2 legs of the triple crown, having won both Monaco and the Indy500, but he hasn't shown any interest in Le Mans or the crown.
Rosberg and Webber have won Monaco, but are both retired. Webber said he's had too many close calls in his career, and doesn't want to push his luck anymore. Rosberg only recently retired, so he might decide to make a comeback later, but for now he looks very happy in retirement.
Hamilton and Vettel have won Monaco, but have not shown any interest in either Le Mans or Indy500. This might however change later in their career.