What year did he do it?
I don't think there has been a human that has been able to do this in the same year.
No cyclist has ever won all three Grand Tour events in the same year. Merckx probably came the closest
Nine riders have hit a double by winning two grand tours in the same year:
Seven have won the Tour/Giro in the same year:
Fausto Coppi (ITA): 1949, 1952
Jacques Anquetil (FRA): 1964
Eddy Merckx (BEL): 1970, 1972, 1974
Bernard Hinault (FRA): 1982, 1985
Stephen Roche (IRL): 1987
Miguel Indurain (ESP): 1992, 1993
Marco Pantani (ITA): 1998
Two have won the Tour/Vuelta double:
Jacques Anquetil (FRA): 1963
Bernard Hinault (FRA): 1978
The Giro/Vuelta double has been done three times:
Eddy Merckx (BEL): 1973
Giovanni Battaglin (ITA): 1981
Alberto Contador (ESP): 2008
Yes, I'm a good "googler".
5 riders have won 5 or more TdF: LA, Indurain, Hinault, Merckx, and Anquetil.
3 riders have won 5 Giros: Merckx, Coppi and Binda.
And the most ever Vuelta wins by the same rider is 3, and only 2 people have ever done that: Tony Rominger in 92, 93 and 94, and Roberto Heras in 2000, 03 and 04. Does anyone think either of them rode clean?
Merckx clearly came the closet to the Triple Crown and would have likely done it in 1973. He was specifically asked by the TdF organizers not to ride in 73 because he'd already won 4 times, and was approaching Anquetil's then-record 5 wins. So he instead rode the Giro and Vuelta and won them both in 1973. We'll never know what might have happened had he ridden all three.
Merckx probably could have achieved more than 5 wins in the Tour had it not been for the "punch" in 1975, and his non-participation in the 73 Tour. He also could likely have won at least 6 Giros had it not been for his doping suspension in the 1969 Giro which he led for 16 stages before testing positive for a stimulant which was later removed from the list of banned substances. He was allowed to ride the Tour that same year after testing positive in the Giro, and won it.
I personally don't think anyone can come close and no one will ever approach Merckx' level of dominatoin. It may be possible for someone to eventually do all three but it's pretty unlikely.