There are many different stories about the making of Zombie 3.
Apparently Fulci shot 40-70(?) mins before quitting (or being fired?) due to several reasons (his worsening health is often given as the main reason/a possible dislike of the project as an other).
Mattei (or Fragasso who apparently stept in as a director with Mattei keeping an eye on things?) not only added footage but also re-shot scenes(?).
Fact is Fulci wasn't happy with Fragasso's script and even tried to change it during shooting with Fragasso protesting.
Btw I know Fragasso (who I like as a director btw) is very much into action and action scenes.
It's not unthinkable the producer wanted to credit Fulci as the director to insure a "bigger name" was attached to the film.
"Fragasso also says Mattei did not like shooting gore scenes and they were shot by Fragasso in this and all there other collaborations"
"Mattei also finished ZOMBIE 3 when original director Lucio Fulci fell ill. According to many conflicting stories (the actual story may never be known) Fulci was disatisfied with the footage he already had and walked away from the film as he did not want to do the film in the first place. Fragasso says Fulci was sick and thought he had shot a full feature when in actuality he only had around 45 minutes. So Fragasso got Mattei to finish the film."
(Il Commisario/Lovelockandload)