In season 3 only MacLachlan is credited as 'Starring' on-screen.

I checked the videos and I suppose that's true. Interestingly enough season 3 has no introductory cast credits, only a few primary crew mentions, with all cast mentions coming during the closing credits; Kyle's in standalone large print under the 'Starring' heading. Seasons 1 and 2 have introductory cast mentions in addition to the closing credits' cast mentions. At a glance it looks like the season 1 and 2 regulars listed on TMDB are strictly adhered but there might be a few that are left out, such as Grace Zabriskie and Al Strobel. It's a hugely popular show and I know that die-hard fans have probably already labored over this and settled on the right thing. I guess I'm just triple checking for any cracks.

Just because the show is old, popular, and has a huge fan base doesn't mean that the listings here are correct. I already noticed a few oddities and mistakes just from the first glance. Like Sheryl Lee being listed as the same character twice in some episodes (as regular and as a guest), "Katherine Martell" was written with "C", despite that she's credited with "K" in the first episode, etc. Also, there are a lot of blanks, where some minor roles are listed without the character, which means they were not credited on-screen, so probably the credits were copied from IMDb or something because all minor roles were credited on-screen in the show.

But regarding Zabriskie and Strobel, as far as I remember she was only credited as "Also Starring" once, in the pilot episode, and he was never credited as a regular.