First, let me say the rating of three is probably a little unfair. I'm giving that to the book based upon it strictly as a standalone product with the buyer only having the Savage World Core rules to go with it. By that guideline it has both pros and cons and should be considered adequate. Taken in a different light as part of a the greater Savage Worlds setting it has potential but more on that later.
Based on the three rating here's how I view it.
The Good: An underused period with plenty of potential the book does a pretty decent job of providing a GM or player with everything they need to go on a three musketeers style adventure. The setting is very fleshed out and the book provides options for varying it between more historical to an even higher degree of magic. The magic system is interesting and (I'd argue) appropriate and the different styles of sword fighting are a very appropriate addition, perhaps even the best implementation I've seen for a swashbuckling type setting. Artwork in the book is both of good quality and could be useful in game. The map of Paris and the provinces of France in the back is a very nice addition which I'd argue is generally useful for anyone running a musketeer or pre-modern setting French game.
The bad: Unfortunaty I can't say it really comes together well enough. The book does an EXCELLENT job of giving you potential plot seeds and has one very nice central twist (which I'm not going to spoil) but it doesn't even have a bare framework of starting a campaign despite saying all your characters must be musketeers. I'm not going to disparage their excellent work by saying that this needed a start to finish plot point campaign but at least something to get the players started and give the GM something to build on would be nice, especially if you're going to railroad them somewhat on the character building front.
This goes hand in hand with all the world building and details, both historical and semi historical. I like having them and I know there are GM's out there that would prefer this method to taking up space with official adventures but I would have suggested cutting down on some of the details and providing at least one or two adventures just to show how it could be done and provide more of an example of how to do certain types of musketeer adventures.
The book could have used some more proof reading, none of the mistakes were 'what the #$%!' level but there were a couple of confusing ones and at least one that qualified as pretty bad, replacing the whole opening paragraph describing the Rosicrucians with something else entierly.
Conclusion: All in all none of this were deal breakers but I feel I have to put the book at a 3/5 because of them rather then something higher.
Now, having said all that, taken as something larger this book has a LOT more potential and could really shine for some GM's and groups. The dark Savage Worlds outlook has always promoted interconnectivity and using this book in something like that is really promonent. I can't remember who (Mark Vassal?) suggested that an interconnected campaign against the forces of evil over time through various Savage World settings is a very real and cool possibility and this setting would fit in well. More then that, looking at some of the characters and groups you could well use the things from this book to determine heroes and villians of the whole overarcing story.
Even if that isn't your cup of tea both the magic system and the fencing schools could be adapted and included into other settings. Maybe it's just me but I think Iron Dynasties is screaming for a GM to lift the fencing schools and their included rules out of this book and put them into that world, you could almost do it with just a change of names. I'm not entierly sold on some of the rules and the complexity they can add to combat BUT that's just me and I can still see plenty of potential there. The magic sytem isn't quite so great but it provides an interesting alternate option for magic settings the power point rules don't always work in. If your players want a higher powered Solomon Kane magic system of perhaps something different for the Weird Wars setting, I could see this working with a few tweaks. A word of warning to GM's though, a stricter penalty of some sort for failure might be in order, I kept having visions of my players trying to cast spells of escape time after time even though they were bound and gagged.
So to sum up: an adequate product by itself, some good ideas marred by some glaring gaps and failures in editing. Taken together with other Savage World books it might be something more and maybe even qualify as a must have for certain GM's and play styles.