Triples have been used in pro races, albeit infrequently, over the years. Giovanni Battaglin won the 1981 Vuelta after he used a triple on a crucial climbing stage. (There are a number of more recent examples, but this is the first that came up in a quick search.)

From this Cycling Weekly article:

"At this point in Battaglin’s 1981 Grand Tour campaign, with the likes of Silvano Contini, Giuseppe Saronni and Tommy Prim snapping at his aching heels, his legs might have decided enough was enough.

After all, the Vuelta didn’t have a single rest day. But Pinarello had a secret weapon waiting for him: a climbing bike designed especially for the Tre Cime stage equipped with a custom triple chainset that would enable him to spin up the ramp-like final climb, while his overgeared rivals burnt all their matches as they laboured up in pursuit."