I usually break omnibus e-books into parts to read and review. But the total size of this trilogy is only 450 or so pages, so I read them all together. That has pluses in that it's easy to follow the path of the story, but minuses in that the stories are somewhat distinct (and can be read independently).
Exiled from Earth
While all three books were written in the early to mid 70s, the first is the one that reads most like it (or even earlier). The hero is strong and capable, the woman is delicate. There's a very nice touch regarding a gorilla, but overall it's a fairly standard adventure story. There's nothing wrong with it, but there's also nothing in particular to recommend it. The writing is smooth (allowing for the change in times), and the plot is thin but adequate - the world government decides to preserve stability by imprisoning (in an orbital habitat) all geneticists , because some have discovered a way to extend life.
Flight of Exiles
This is a love-triangle mystery set around the arrival of the habitat (now implausibly converted to a spaceship) at a new world. The science fictional elements here are really just backdrop for the personal drama. The story is decent, fuller than the first, and the characters more rounded (and less trapped in their era). Still, I'd either read this book before, or read one just like it by someone else; it had a very familiar feel to it.
End of Exile
Clearly the best book in the trilogy. It's a variation on Heinlein's Orphans of the Sky, about a generation ship that has forgotten its purpose. It's not as involved as Heinlein's story, but is more hopeful and approachable. It's weaker on personal relationships and credibility than book 2, but manages to be the most moving and involving of the set. If you're buying the books individually, I can safely recommend starting with this one, and reading the others for background if you want to. If you buy the full trilogy, rest easy that they get better as you go.
All in all, a decent trilogy, but only the last book is really strong. If you find the trilogy for a low cost, that one is worth the price of entry. The others are a good way to pass some time, but won't change your life in any way.