Season 3 was a big disappointment for me. Some great moments (tears came when Uthred talked of his relationship with Alfred), but overall, the plot didn't make sense.
I understand the need of condensing the plotlines of multiple books, but I thought it was better handled the first two seasons. For instance, in season 2, they gave the Norse brothers (book 4) the role of Ivar and Ivarr (book 3) which was smart and allowed to have them present the whole season. Otherwise, the show still followed the events of the books pretty faithfully.
In this season, they had the so called new Great Army waiting the whole season for Alfred's death, which is just absurd, both from a storytelling point of view and in-story logistics (the army would just starve and disband before that).
But mainly, I disliked:
Heasten and Aethelwold's back and forth with Wessex, it required way more suspension of disbelief that I could afford.
the original ending they gave to characters that just vanish in the books. Ragnar in the books was original as a Viking as he just wanted to live in peace for the rest of his days in his castle. And he did. It broke my heart to see the death they gave him in the show. Same for Thyra. Why couldn't she just live happily ever after with her Beocca and their many children, as in the books? Why? Was it necessary to have the whole Ragnar family die horribly? And poor Beocca to live trough this...
The whole Skade fiasco. Keeping her for much of the season was their biggest mistake.
And some other less significant nitpickings, like the mid season battle that was atrociously directed and horribly written (really, Alfred, you want to have all Uthred's men killed just to make a stupid point?).