As mentioned, being a kid makes Gym Battles much harder, but that being said the Gen 3 Gyms are more difficult than any other Gen.
If you picked Treecko you were basically playing on hard mode.
Roxanne is super easy.
Brawly is only difficult if you don't have a Taillow, Zubat, Ralts, Kadabra, etc. So for a first playthough he might be a challenge.
Wattson was easy if you picked Mudkip, if you picked Torchic it was average.
Flannery is easy.
Norman is hard if you don't bring a specific counter and aren't overleveled.
Winona's Altaria can be difficult, with it spamming EQ.
Tate and Liza were difficult, but I seem to remember their team was easier in Emerald.
Wallace was no problem.
Also tons of people said that had a problem with Juan, which I didn't understand until I remembered like the lazy fuck I am I caught the Level 70 Rayquaza and used it to beat the remainder of the game. Well, they were the ones that made it catchable before you beat the Elite Four.
Whitney, Clair and Morty were difficult when I was a kid, Morty and Clair to this day aren't exactly easy.
Fantina and Wake were a challenge and so was Maylene, but that was because I played Gen 4 blind and didn't know Lucario's type, so it would sweep my team until I tested enough types to figure out it was Fighting/Steel.
Anything else was a breeze.
