I personally would have used Monaco, Silverstone and Monza in one season as the Triple Crown.
If that was the case the only winner would be Prost 1985.
But if you do that in your whole career here is the list of winners:
Fangio: Monaco (1950, 1957), Monza (1953, 1954, 1955), Silverstone (1956)
Stewart: Monza (1965, 1969), Monaco (1966, 1971, 1973), Silverstone (1969, 1971)
Prost: Monza (1981, 1985, 1989), Silverstone (1983, 1985, 1989, 1990, 1993), Monaco (1984, 1985, 1986, 1988)
Senna: Monaco (1987, 1989, 1990, 1991, 1992, 1993), Silverstone (1988), Monza (1990, 1992)
Schumacher: Monaco (1994, 1995, 1997, 1999, 2001), Monza (1996, 1998, 2000, 2003, 2006), Silverstone (1998, 2002, 2004)
Coulthard: Monza (1997), Silverstone (1999, 2002), Monaco (2000, 2002)
Montoya: Monza (2001, 2005), Monaco (2003), Silverstone (2005)
Alonso: Monaco (2006, 2007), Silverstone (2006, 2011), Monza (2007, 2010)
Vettel: Monza (2008, 2011), Silverstone (2009), Monaco (2011)
Hamilton: Monaco (2008), Silverstone (2008), Monza (2012)