There's a difference between 1 v 3 separately and 1 v 3 where rhe 3 are actively working together to take you down
They were actively working to take him down.
They didn't try to fight all at the same time because they were scared they would get taken down all at the same time. It was a way to try hurt him without taking lethal damage across the board.
Think of it like fighting a Legendary Pokemon with your shitty common pokemon. They might be elite pokemon, but you still end up shifting them out whenever one gets the crud beaten out of them.
That's exactly why they kept shifting out and taking turns.
Sengoku was the trainer.
Magma pokemon level 50 = Akainu
Light pokemon level 50 = Kizaru
Ice pokemon level 50 = Kuzan
Legendary Quake pokemon level 60 = Whitebeard
That's basically what it was.