Clara and Koldo are getting married, and the first part of the movie is shot as if you're watching a wedding video from the POV of the camera guy. We see that an Uncle is sick and learn that he was bitten by a dog. He gets more sick until he falls off a balcony at the reception and then attacks a woman who goes to help him. Almost immediately she's infected with the zombies as well, as are the majority of the wedding goers.
The bride and groom get separate and have to find each other This is when it stops with the POV wedding footage and becomes a narrative film. The rest of the movie is the lovebirds trying to reunite. What's interesting is that the zombies here are caused by demons, not a biological virus. They don't like crosses, hearing prayers or passages out of the bible.
Part 3 is a great prequel to the first two REC movies. Highly recommended.