First of all DS9 was a Trek show that first introduced a three-parter.
I'd argue that TNG had the first Trek three-parter with The Best of Both Worlds I/II and Family
. And those episodes were part of a bigger arc starting with Encounter at Farpoint
about humanities place in the universe.
And I would argue that not Berman, not Piller nor any of the promotion department folks at Paramount EVER advertised those episodes as a three-parter. That would be just as silly as saying "In the Hands of the Prophets", DS9's first season finale that led to the three-parter that opened its second season, was actually the first chapter
of a four-part
episodic run of DS9. But no one here is silly enough to do that.
Go take it up with both Berman and Piller who in media promotions/interviews declared DS9's second season The Circle Trilogy as the first three-parter of Star Trek. This was how it was reported in the pages of TV Guide, USA Today and now defunct genre magazines. TPTB was trying to regenerate buzz for DS9 and so they intentionally came up with the idea of doing a three-part storyline for the first time in Trek history to open up season #2.