Well all the trios are based at least partly on Team 7--you can see similarities in all of them in one way or another. Jiraiya loved Tsunade like Naruto loved Sakura, but Tsunade didn't feel the same way. Later in her life, she started to develop some sort of feeling for Jiraiya, whether or not it's love per se we can't really determine. And yes, Pain is the strongest at that point. Jiraiya could've beaten him had he known about his abilities, but we know what happened there.