It was common for TV shows in the '60s to center on only one or two leads and have everyone else be recurring/supporting players. Originally TOS was very much a star vehicle for William Shatner -- already an acclaimed rising star for his stage and screen work -- but Leonard Nimoy ended up stealing the show and getting the most fan mail by an enormous margin, so the network pushed for him to be more centrally featured. DeForest Kelley also proved popular enough for a promotion to the regular cast.
But if you look at the end credits for the early episodes, Grace Lee Whitney (Yeoman Rand) was generally billed fourth, after Kelley. Takei, Doohan, and Nichols were listed after them in the guest credits (under "And"), and not in a consistent order. Once Whitney left, the others started getting billed more prominently, though not consistently. And the series never had more than three official regulars, Shatner, Nimoy, and Kelley.