It seems almost a shame to admit that the third book is a little weak. Or, I don't know, not as strong a resolution to the series as I'd hoped.
The Venus Mystery pops off of Venus, flies out to Neptune, and turns into a stargate. (Gedge as much as Emmerich, we will discover.) So now what? An international expedition is organized; "international" means Earth, Mars, and the Belt, among whom there is no more trust than there is mayonnaise in a supernova. All our old friends from the first book are along for the ride. (Our friends from the second book get cameos, no more. Apologies to UN-undersecretary fans.)
New viewpoints: a pastor and someone who is very, very angry.
What does the trilogy look like? The first book used alien biohorrors to explore how horrible humans could be to each other. The second book used even less alien; just alien seasoning, as it were, in a political drama. The third book tries to up the stakes by throwing everybody through the stargate, and having them be human beings there. This... almost works. The human/political drama is still there, but we are learning more about the aliens too, and that unbalances the story. In my opinion, I mean! It's not a total failure; in fact it's not a failure, but it is a weakness in the story. The big climax comes off as "We must push this button to solve all our problems." Getting the button pushed is a real story about real people -- with politics, mutiny, and gunfights -- but I just didn't think it quite had room to breathe.
Yes, I'm this close to saying that the authors should have expanded the storyline into a fourth book. I know! Terrible idea. Three is fine. They're good books. Captain Holden is a great character; the genre has so many stupid optimists, and plenty of implausibly right optimists, but so rarely an optimist who is both right and wrong in smart ways.