"For the World Is Hollow..." is a hackneyed plot - a Star Trek staple plot, for that matter - but well done. The Enterprise encounters an asteroid that isn't an asteroid - it's a hollow, artificial planet, carrying the descendants of a distant race to their eventual new home, run by a computer that overdoes its protective job of caring for them and has gone a little megalomaniacal. The high priestess of the artificial planet - who, like her people, does not know she is inside a hollowed out asteroid, or that her god is a computer - takes a fancy to Dr. McCoy, who has recently discovered (will the cliches never end?) he has only a few months left to live, and as a result accepts her proposal of marriage and retires from Starfleet service to spend his final days with her. Needless to say, Kirk and Spock have to rectify the entire situation. The episode is nicely produced, for how [inexpensive] it is - the entire third season was [inexpensive] - and Kate Woodville is endearingly naive and regal as Natira, the asteroid-planet's priestess/McCoy's new bride. The sets and costumes are quite attractive and colorful.
"Day of the Dove" is great fun, more for its cast and the gusto with which they perform their roles than for the story itself. Kirk and Co. find themselves lured by a fake distress signal to a planet where only a half dozen Klingons survive. The Klingons blame the Federation for having lured them to the same planet with a fake distress signal, and killing most of his crew. After Kirk gets them safely rounded-up and under guard aboard the Enterprise, all hell breaks loose: an unseen power hijacks the ship outside the solar system at Warp 9, in circles, and releases and arms the Klingons and the Enterprise crew with swords; the two rival races fight to the death, over and over again, since the same unseen third party seems also somehow to keep repairing their injured bodies. Kirk, one way or another, has to gain the trust of the Klingon leader to identify and eliminate the alien invader responsible for the carnage, before they are trapped in eternal warfare with each other.
"Dove" is a real scenery-chewer, and one of the [least expensive]-ever episodes of the series. Only the Enterprise core cast and a handful of Klingons are ever seen - everyone else, we are informed, has been sealed off (conveniently and cheaply) below decks - leaving them to roll their eyes and gnash their teeth in artificially induced fury for most of the hour. Michael Ansara, who never disappoints, is ideal as the Klingon captain, Kang, and Susan Howard - in one of her final performances before permanently retiring from acting - is appealing and interesting as his emotionally torn wife, Mara.