No doubt he lost all three times to Nala, though the first two were just being playful. Also, the adult fight was when he had absolutely no fighting experience, while Nala was an experienced hunter.
Against Scar, if a fight were determined by whose back hits the floor first, then Scar lost, since he got pinned by Simba before he landed the blow on him. The hyenas just intervened. Against the hyenas, he wouldn't have lost if Rafiki didn't come, maybe just a slight injury or something. Against Zira, she just got a hit on him because he was fighting against 5+ other lionesses. At the end, they were both going for the attack, meaning they would trade blows. If that were the case, Simba's hit would obviously be more powerful.
