More than the other two seasons, the third has suffered the most from the 2nd half slump. I was never really looking forward to the second half of this season- already knowing what was to come, and it all really seemed to be just a very convoluted way to get the characters to America, where bigger things are to come and other points are set into motion. Up until the reunion, the show was building a great momentum- most importantly emotional momentum. The audience got to delve into what has shaped Jamie and Claire during their years apart, which was compelling viewing. However, after the couple reunited, the plot got its needle stuck on the same reoccurring themes of separation (due to the characters becoming different people or due to actual, forcible separation) and of course the theme of unwavering love (we understand, the couple has still got it). These two themes are usually very effective, but when repeated over and over without any other real plots or emotions at play to support them, it just becomes tedious and predictable; come on, one never really fears that Jamie and Claire will separated again so soon to being reunited.
I know that the show is only following the outline of the novel. However, the main disadvantage the show has compared to the book- as with many adaptations- is that in reading the story-lines, there is a lot more filler and description- in-between moments that separate the many coincidences and reoccurring themes. So, one feels things are a bit more spaced out and evenly paced. I can't quite put my finger on it, but this adaptation in particular seems to suffer a great deal from inconsistent pacing or writing over the course of the episodes and odd moments of focus- mostly moments of intimacy that feel forced or wedged into the flow of the episodes. Just because a small scene happens in the book, does not mean it needs to be placed in the show where time and focus is much more limited and valuable.
Hopefully, next season when more characters and greater challenges take center stage, the show's momentum and success won't fall directly onto the shoulders of Jamie and Claire's relationship (not trying to demean its importance or value). With the full addition of Brianna and Roger, the show can focus on a dynamic more rooted in reality (less perfect or assured), which can break the monotony a bit. Also, a larger role for Lord Grey, whose presence adds a little tension, as well as unexpected support, or even the journey of Young Ian.
Compared to Voyager, Drums of Autumn is, as a whole, a more captivating novel. So, hopefully, season four will be too.