Devon Young did some digging into when the term “Triple Crown” was first applied to winning the BA, HR, and RBI titles.  He writes:

With all the talk that Cabrera might win the triple crown, I thought it’d be interesting to see what people thought of Yaz winning it back in ’67. Some of these are pretty interesting & show how times have changed and that the term didn’t even exist in 1937 when Medwick pulled it off…

Two Super Stars Hound Yaz (Sep 21, 1967)

Yastrzemski Cops AL Triple Crown (Oct 2, 1967; tho Google somehow mistakes it for Sep 23)

Yaz May Get $100,000 Salary (Oct 3, 1967)

…which got me interested in some older articles too….

Robinson Headed for Triple Crown (Sep 27, 1966)

Triple Crown Clout to Frank Robinson (Oct 3, 1966)

Mantle Cops Three Bat Championships (Oct 1, 1956)

[interestingly they only knew of 7 triple crown before this. The end of the article only mentions Hornsby, Klein, Gehrig, Foxx, & Williams. That means they weren’t considering Joe Medwick’s 1937 triple crown as an official triple crown yet, probably ’cause he tied for the league lead in HR’s. Yet some of the ’67 articles on Yaz count Medwick as official. Besides that, they didn’t mention any of the pre-1920 winners.]

So… in 1937, did they think of it as official? Turns out, they didn’t even use the term “triple crown” yet! Some articles don’t even mention he led the league in so many stats categories, but most articles definitely acknowledge he did and that it was rare and impressive. They didn’t have a term for it yet tho. They seemed more fascinated that he led 4 batting categories (including doubles) than simply leading the triple crown categories.

Gehringer and Medwick Clinch Batting Titles (Oct 3, 1937)

Gehringer, Medwick top (Oct 4, 1937)

Ducky Medwick Corners Batting Honors For Term (Oct 9, 1937)

Gehringer, Medwick, Top Records in Batting Averages (Oct 9, 1937)

…and Medwick’s feat seems to be what makes writer’s begin using the term “triple crown”, as just 2 years later, they’re writing things like “Dimaggio seeks triple crown” (Sep 2, 1939). It’s used pretty often after that, especially in ’41 & ’42, in reference to Dimaggio or Williams.

In 1933, when Chuck Klein won the triple crown, I couldn’t find any articles that seemed to think it was nearly a big deal, just impressive. Like Klein Garners Batting Title (Oct 1, 1933), which focuses mainly on his winning the batting title but briefly points out he was also good enough to win some other batting categories. Not nearly the big deal anyone would make out of it these days.

In an August 4, 1929 article, used the term “triple crown” referring to the player with the most triples.

Before that, I couldn’t find a hint of the term.