How is it possible for Trunks to fight with Goku Black, and hold his own better then either Goku or Vegeta?
Dragon Ball is known for inconsistencies and plotholes so it might simply have to do with something of that. But aside of that, some reasonable arguments I've seen online are these 2, each time a saiyan is knocked down it gets a senkai boost when it recovers (increase in his power) . He was knocked down two times, by Goku and Vegeta. In the manga, when he is fighting against Goku, he is said by Vegeta to be almost as strong as a Super Saiyan 3 . It is possible for a Super Saiyan 2 to be as strong or stronger than a Super Saiyan 3, Beerus says Vegeta (in Super Saiyan 2) made him use more of his power than Goku (in Super Saiyan 3) when fighting against him. After fighting a Super Saiyan 3 (Goku) and being knocked down, he recovers as a saiyan to catch up his power, and later he is knocked down by a Super Saiyan Blue (Vegeta), and he recovers as a saiyan to catch up his power. So the argument says that when he might not be as strong as a Super Saiyan Blue, he might not be as far of it as for being unable to step in fresh by surprise in a fight, and score some succesful punches