It does not have to do anything with the first 2 parts except the infection. It starts as a found footage movie and then switches to a standard shot movie. The movie itself is like a typical zombie movie and misses the terrifying moments from the first 2 parts. It is not scary at all but it is still a good and watchable zombie movie.
Synopsis - SPOILER ALERT
The wedding of Clara (Leticia Dolera) and Koldo (Diego Martín) should be like any other, but suddenly the uncle (Emilio Mencheta) of Clara has to vomit on the terrace and the special commando with protective suits storm the grounds and panic breaks out. A short time later, the uncle attacks his wife and bites her bloody. What many don't know, Clara's uncle is a veterinarian and treated the little girl's dog who brought the infection to the apartment block from the first two parts, which plays parallel to this scenario. More and more guests are bitten and become infected with the virus. The hotel where the wedding is taking place will be quarantined. From now on, the guests and the couple struggle to survive.