Due to the close proximity of the Vuelta and the Giro, it was rare for people to race all 3, nevermind win them.
I think that the highest ever finisher in 3 GTs in a year was Frederico Bahamontes in 1958- 17th in the Giro, 8th in the Tour and 7th in the Vuelta.
However, I may have been misled on this issue.