Nagato would win with mid to low difficulty. He took on Bee, Naruto, and Itachi, all of whom are stronger than the Sannin and he would of won too if Itachi didn't have Totsuka. It also doesn't seem like Orchimaru nor Tsunade have a jutsu that could break the gravity ball from Chibaku.
we're not talking about Ultimate Nagato, just healthy Nagato, (which can be safely said) is weaker than "Ultimate Nagato". Therefore, your argument is what?...invalid. and you forget about the summons. hardly one man can take on a team of 3, especially the sannin. who knows, even hanzo may have lost if the battle ensued. but present tense, no way Nagato would survive. do not forget, oro has aimed to learn all jutsu. so there's no telling what powerful jutsu he may have learned. and when it comes to CT...:
slug summon to heal...gamabunta + jiraiya combo attack...oro + edo tensei, 1st hokage practically could stop the attack alone, let alone add the 2nd's mass water power.
so that arguement is invalid as well.
and i don't understand why anyone would stand around and watch this big ass ball be formed...without the paths, nagato's vision is limited. too many distractions for him.
all in all, the sannins would win; ppl are highly underestimating them and highly overestimating Nagato. #thatisall