Naruto, Sakura, and Sasuke were standing back, and Kakashi was fighing Zabuza all by himself. Why?
At that time, the members of Team 7 has just passed from the Academy, they were just kids. Sakura couldn't help much because she was very weak, she hadn't learnt medical ninjustsu to reinforce her punches. Sasuke may have been a bright child but he was still an amateur and he hadn't awakened his Sharingan. Even if he had awakened it, Zabuza was a formidable opponent, one of the Seven Ninja Swordsmen. And Naruto didn't know any jutsu other than Shadow Clones, and he didn't have control over Kurama's power. But Naruto and Sasuke did fight against Haku if not Zabuza. It's obvious that the strongest team member Kakashi should fight against Zabuza since he's so strong.
P.S. I think you have not watched the Naruto series and you've only watched the Shippuden series.
If you are talking of the very first fight that happened between Zabuza and Team 7 then you are talking of the incident when they were still escorting the builder back to the 'Land of Waves'. In that case there are a couple of things to consider. First, the trio of the Team 7 genins were fresh out of the academy and this was their first mission which was meant to be C or D class but turned to be a A class mission. Second, Naruto might have just taken the oath to never fallback from a fight when they were attacked before by the brother assassins from the Mist but the essence was he was still a child who could not control his chakra well and only had one proper move which was the Shadow Clone Jutsu. At the same time though Sasuke was smart and talented, his experience was quite lackluster in front of Zabuza. As for Sakura, she was not at all in the race to begin with, even thiugh she had better chakra control then the other two. So it made sense that Kakashi fought one on one with Zabuza. Even after that though it was not as if Naruto and Sasuke did not lend a hand with their combo-sneak attack by helping Kakashi Hatake out of the tough spot when he gets caught in the Water prison of Zabuza's creation.