Interesting historical fact: In the 1940s, DC contracted out to a second publisher, All-American comics, to produce more comics under the DC brand. The top three characters at DC proper were Superman, Batman and Robin (starring in World’s Finest). The top three at All-American were Wonder Woman, Flash, and Green Lantern (starring in Comic Cavalcade). In the middle of the decade, DC bought All-American outright.
Those six characters are still the most recognizable from DC, but… Robin is always the second half of “Batman and…” Green Lantern and the Flash disappeared for several years before being completely reinvented in the late 1950s, and have never reached the level of mainstream recognition that Superman, Batman, and Wonder Woman have.
That’s what I think matters here: the “trinity” are the three DC characters best known to the American public at large over the last 7 decades, not those who are most popular among today’s comic book readers: Those would be Batman, Batman, and Batman