Planet X was a stand alone, actually. You do NOT have to read the crossover comics.
Well, no, you don't really have to, but it does make some things a bit clearer if you read the TNG crossover comic. From the X-Men's point of view, the novel takes place immediately after the cliffhanger ending of the comic (though substantially more time has passed for the TNG crew), and the book references the events of that comic, since the X-Men and the
Enterprise crew already know each other from that prior adventure.
So it's not a question of "need," but you do get a more complete experience if you read the comic and the novel back-to-back. You don't "need" to know the specifics of how Picard's crew and the X-Men know each other, since everything relevant is explained in the novel, but if you're
curious, the comic would answer your questions. People insist on defining these things in terms of "need," but it's not about whether you need to know, it's about whether you'd be interested in knowing.
As for the earlier TOS/X-Men crossover comic, it doesn't really connect to the two subsequent crossovers, although its events are alluded to briefly in the TNG/X comic. (The X-Men teams in the two comics are different, with only Wolverine and Storm in common.) As for myself, though, the TOS/X comic is my favorite of the three crossovers.