Gen 3 as a whole was a really big change to the mechanics of Pokemon and I don’t think people just realise how many Pokemon were introduced to showcase them so here are some of my favorites :D
Pokemons like Slaking and Shedinja are obviously meant to showcase Abilities but what most people overlook is that this applies to the Weather Trio too :o Think about it, their Abilities are basically their whole gimmick and identity to the same extent that Z-Moves are for the Light Trio and Paradox Forms to Koraidon and Miraidon. If they were introduced in Gen 2 and then gained the Abilities in Gen 3 there’d probably be a False Swipe Gaming video called “Broken Pokemon that got EVEN stronger” or something
In Gen 1 and 2, a Pokemon’s “genetics” (gender, shininess, etc) were determined by their Determinant Values (basically just IVs) which led to some funky stuff like how a Pokemon with a 1:7 female to male gender ratio can’t be female and shiny at the same and two Pokemons with Identical DVs in certain stats can’t breed. In Gen 3 they completely overhauled this with Personality Values which assigns a random number from the 32-bit integer to the Pokemon. Some Pokemons that were intended to showcase this are Wurmple’s evolution and Spinda’s spot patterns :D
These next ones are a little different because the Pokemons themselves weren’t designed to showcase Contests but all the in-game trades in Gen 3 have enhanced stat in a certain Contest Condition (which explains why they’re not good for a playthrough lol :> (well the trades in RSE sucks but FRLG are just like most game : some Pokemons are good but it’s still a mixed bag))