I would turn the mushroom toad away from frogs because of the similarity to Beelzebufo and Bulbasaur. I would maybe go towards... hmmm... Perhaps some sort of ceratopsian. Yeah, you could make the mushroom cap turn into a big shield in the second stage, give it a tail, and put some spines down its back, then in its final stage give it a couple horns, make it larger, and make its spines on more than just the spine itself. Ceratopsians (like triceratops) had legs kind of like a frog, with short in the front and pillar-like in the back. You could also incorporate some sort of, as I said, shield-style or armor imagery into the design, and end with it being Grass/Steel.
For the snake, I think an easy - if simple - choice could be Quetzalcoatl, but that's kind of done to death. Instead, maybe you could make it into a mystical oriental dragon, perhaps also tying it into some sort of magic staff as a motif or something, or maybe with fireworks. I like the fireworks idea way better after thinking about it. Actually, maybe gunpowder in general? Maybe pure Fire, maybe Fire/Flying, maybe Fire/Ground?
For the slug, you can obviously follow Glaucus Antlanticus, but I would try and make it a bit different. For one thing, I suggested subthemes of "armor" and "gunpowder" for the previous two, so how about something also about afvancements in warfare - maybe nuclear bombs? With a slug, it should be pretty easy to add nuances of design into it, and what I'd personally do is maybe take it in a kaiju-type direction, with the mid stage sprouting legs and maybe resembling Godzilla or some other sea monster in general or something, but making the third stage its own thing. I would be careful to a) avoid making it similar to Goodra b) avoid making it "godzilla but a slug" and c) play up that it is a sea slug. Either way, you could easily incorporate a radiation logo or similar on its back or something whatever you pick, and I feel Water/Poison would be the most obvious typing.
