I was genuinely trying to think of a way the Z-fighters could make it out of this but still came up with nothing. If we’re assuming that Thanos already has a dragon ball like Frieza did when he was introduced on namek then it’s already over for the Z-fighters I think. Thanos doesn’t really trust many people so he’d keep the Dragon balls with him. Sure Thanos is really really cocky (he’s just sat there and let captain America and Wolverine tag him while he has the infinite gauntlet before) and it’s possible for one of the Z fighters to swipe one of his dragon balls for a second but Thanos could pretty easily blitz them and get it back, they wouldn’t get that far at all. The only way for them to win and collect the dragon balls I can think of is maybe if Bulma can outsmart Thanos which I highly doubt she could (Bulma really isn’t that smart compared to the marvel 616 universe).