Tsunade= Orochimaru= Jiraiya [READ]
Slug, Snake, Toad
Sounds familiar? Yep, those are the summonings of the Legendary Sannin - Jiraiya, Orochimaru and Tsunade.
Why these three animals? There is another reason from Japanese legends. I'm sure everybody knows the game "Scissors, Paper, Stone" (also known as Janken in Japanese) where the scissors cuts paper, paper covers stone and the stone smashes the scissors. The Slug, Snake, Toad works in the same concept.
In this variation, the Frog is afriad of the Snake, the Snake is afriad of the Slug and the Slug is afriad of the Frog. This is influenced by the Sansukumi way of thinking where it keeps the 3 objects in a deadlock. The fact that the Sannins use these 3 animals as summons may be the author hinting that there is no stronger one among them, and that their strength are more or less equal.
According to this they are more or less equals. Tsunade ( Slug) > Orochimaru ( Snake) > Jiraiya ( Toad)> Tsunade ( Slug), etc.