In the games, it would have been hard to fit three or more types onto a Pokémon, so they didn't. Instead, they later came up with "abilities" that fixed the problem... to an extent.
It's already been estalished that many of the attacks Pokémon use are just plain magical. How does a 5'3" tortoise spew an olympic swimming pool's worth of water every minute seemingly from nowhere, how do these electric and fire sacs work, how does a cat punch and make weird elements and objects come out of their paws, etc. Such questions can be "answered" by saying that some Pokémon and attacks are just supernatural. However, which attacks can only be natural? How would Pokémon be different in order to fit the real world? What about their typing?

In reality, some Pokémon would have different types. Some could have more than two types (Beedrill and Flygon would be part Flying, Charizard and Gyarados could have Dragon added to them), and some could lose or gain different types (would the Flyers and Girafarig really be part Normal? Should the Gastly line be part Poison any more than they should be part Dark? Why should Cubone, Marowak, and their bone attacks be considered Ground types? Gyarados can't fly... but Dragonair can...).

Bulbasaur can throw leaves from nowhere (or perhaps its back pocket?). Pikachu can call up electric storms. Diglett could spew sludge that it probably doesn't normally have. However, it's unlikely, from what I remember, that an Onix could throw rocks it doesn't have, a Ninjask could use Sand Attack in the middle of the ocean, and that Totodile could pull Surf in the middle of a dry woods. Where's this going? I don't know, I just found it interesting. But one thing I wonder is if some attacks would be reclassified. Megahorn just seems to be throwing the opponent with one's horn. Could that really be called a "Bug" attack, and would it really be strong against Psychics? Same question applies to Fury Cutter. Mirror Move seems like a short-term Mimic. Only bird Pokémon can use it, but it really has nothing to do with flying. Though I understand the origin, Grass-type attacks don't seem like they would actually harm Rock-types any more than they would the other types. What types would change and how? How would attacks change?