As posted by Fabrizio, Nottingham Forest has parted ways with it's manager Nuno Espirito Santo, these's was a tense situation at the club between him and them, I believe that's the main reason why he was sacked. Nottingham Forest has started off this season with a win, a draw and a lose in three matches. I believe that's one of the major trigger as to why he and the club management has fallen off and it resulted in a sack.
I don't believe Nuno Espirito was fired because of the results in the last three Premier League games, it's only the start of the season, and they can't use it to judge a manager who managed the team last season. If it had been a new manager who had failed to win any of their previous three games; they could have suggested firing him, but Nuno has been in charge since last season. He has been having a misunderstanding with the club owner since last season.
Remember that Nuno Espirito took over the club when they were 17th in the Premier League. Nottingham Forest made it to the top four previous season but dropped back due to some losses at the end of the season. There was a rumour that the club's owner fired Nuno in order to bring in Ange Postecoglou, who was fired by Tottenham after winning the Europa League.