OMG I remember SO well when Gen.3 came out!!! Before that, I have been playing the R/B/Y and G/S/C series for ages xD I was stunned to play my Gold version on a Gameboy Advance for the first time (saved money for my forst console I have bought myself), and seeing all those colours! I was mindblown xD But, as soon as I got word Gen.3 came out, I was amazed by the graphics! Also, I really like the story in there and I think a lot of Pokemon in these games are awesome! I was sold as soon as I saw the new starters (especially Torchic and Mudkip) and could NOT wait to play these games!
When I finally owned Ruby, I remember walking in the beach sand for hours because of the fact you actually left your footprints in there!! And you could see your reflection in the water!! This was totally new for me :3 Also, you could grow and plant Berries, even got RUNNING SHOES so goodbye walking slowly! xD and OMG don't get me started about the Mach and Acro Bikes!!! :D I remember trolling my game to choose the Acro Bike, and go Jumping on my Bike (instead of just cycling), all the way to other Towns and Cities just... for the heck of it! xD (and come on, it was just HILLARIOUS).
Also, Gen.3 was the first gen to have introduced Contests with your Pokemon in a way you feed them something, increase an element such as Coolness, and then use them in a Contest against other players and win Medals :)
OMG and, most fun part imo, the SECRET BASE! xD It was just... too much fun to make your own secret base somewhere, stuff it with neat furniture and cute items (Slide!) and even share them with your friends so you could have each other's secret base in your game! :D I had sooo many great times with this!! <3 :3
Also, as I discussed in other several Threads, you got the move Dive at one Point!!! I love this move, and even today, it remains as one of my favourite HM moves, come on we could Dive into the sea for the first time!! :o Amazing, loved the theme song in there and I spend HOURS on the seafloor, searching for Pokemon and just... swimming around underwater for as long as you like, just because you could! xD
Sigh... soooo many thing so talk about! Well, last but not least you can say I liked these games so much, that I even bought Sapphire and Emerald after that! :3 And still play them from time to time till this day ^_^!