All of the Sannin are based on Japanese folklore. It is similar to rock, paper, scissors. Jiraiya was the toad, Tsunade the slug, and Orochimaru, the serpent. The serpent beats the toad, the toad beats the slug, and the slug beats the serpent.
The speculation comes from that fact. It makes no sense to not only create those characters, but to go as far as creating their own sage mode, respective to the animals they are based on. We know Orochimaru did snakes, but couldn't do sage mode because he didn't have the proper body for it. We assume that there is a slug sage mode, because it fits. Why only do 2, and not 3. Tsunade hasn't shown any signs of being a slug, but we know that Hashirama is her grandpa, and an even greater medical-nin than she is. We know Hashirama is a sage. The most likely candidate would be the only one of the 3 legendary figures, so they'd all be represented.
No, we don't know if that's the case. It may very well not be true, but speculation doesn't hurt, does it? If someone is saying that they KNOW it's the slug sage, they're liars. Only Kishimoto knows that at this point.