Just finished Crucible: Kirk, so I've now read all three.
I'll first say that while all three books tell good stories, it may have been a mistake to make them a trilogy. Continually tying everything back to "City" felt far more forced than it should have; and the repeated scenes grew tiresome. Showing a single scene from multiple points of view is fine once or twice, but enough is enough......I don't need every single version of Kirk running around reminding us via his thoughts every other chapter that Korax blew up the Guardian. I read it once already in this very book, never mind the last one!
Now, taking the individual stories in isolation from the "trilogy" aspect, as I feel they should have been to start with.....
The McCoy novel was fantastic. Even if letting him die just after "Encounter at Farpoint" messed up a bunch of other novels, it still felt like an epic tale worthy of a top shelf position.
The Spock novel, as I've previously said, was a great character study (when it wasn't busy reminding us of things we already knew from the TV show or McCoy novel). Its distinctness from the more "common" noncanon events of Spock's live was a bit jarring, but not a deal-breaker by any stretch.
The Kirk novel felt like a short story padded out to a full book, honestly. A great short story, to be sure, but that was the impression I got. I really don't care about Antonia, so that didn't help. We knew that ended badly already, and the story didn't really give us much to work with beyond that, so I have to wonder why spend time on it at all. The temporal mechanics were fun, though. I did regret not seeing the "Don't you read history?" line in either of the two Veridian III sequences, thus making it unclear which of them we were "supposed" to have seen in Generations. After spending so much time re-telling scenes elsewhere, it seemed strange not to highlight the Veridian III differences the second time around. Also, with all the power of the Nexus available to Kirk, he couldn't come up with a better plan for his other-self than dying the same way he did? Why not encode a time-delayed message for Picard warning him about Soran, and leave it sealed (under the Temporal Prime Directive) not to be viewed until after Amagosa goes up?
Final grades:
McCoy: A+
Spock: A- for the original story, B for the whole book
Kirk: B+ for the original story, B for the whole book
The Afterword said you came up with the McCoy concept first. I'm afraid I could tell.