Sarutobi chose Minato to be his successor, because although he wanted Orochimaru to be the Fourth, he knew that he would only use the title for his own personal gain. It had nothing to do with who was more powerful.
First of all, don't forget that the Sannin each have their own summon. That means Gamabunta (though I don't know who between Jiraiya or Minato would summon him first), Katsuyu and Manda against something else that would Minato would summon. Orochimaru has the Five Elements Seal, meaning if he struck Minato with it, Minato's chakra flow would be heavily irregular, thus preventing him from normally using his power. That's one threat; next is Tsunade's monstrous strength and her amazing healing ability - if Minato struck her with a Rasengan, I'm assuming she would recover, though probably be injured some in the process.
And now, moving on to Jiraiya - he is the one who knows Minato the best out of the three of them. He knows how to use Rasengan, Minato's own jutsu, and has knowledge about the other techniques that Minato shown during training or at some other point in time. Also, he has a vast arsenal of techniques, and although Minato is the fastest, avoiding all the attacks of Tsunade, Jiraiya, and Orochimaru - that's just a little difficult, even for Minato, in my opinion.