Let’s look at the first 3 gym leaders’ Ace Pokémon.
Roxanne
Nosepass
Ruby and Sapphire:
>lv 15
>Tackle, Harden, Rock Throw, Rock Tomb
Emerald:
>lv 15
>Item Oran Berry
>Block, Harden, Tackle, Rock Tomb
Omega Ruby and Alpha Sapphire:
>Lv 14
>No held item
>Tackle, Harden, and Rock Tomb
They removed Rock Throw and nerfed it by 1 level from RS and removed the Oran berry from E. Most importantly it only has 3 moves instead of 4.
Brawly
Makuhita
RS:
>Lv 18
>No held item
>Bulk up, Knock off, Arm Thrust, Sand-Attack
E:
>Lv 19
>Held item: Sitrus Berry
>Arm Thrust, Vital Throw, Reversal, Bulk Up
ORAS:
>Lv 16
>No held item
>Bulk up, Knock off, Arm Thrust, Sand-Attack
It’s the same Pokémon from RS except it’s been nerfed by 2 levels. Since the difference in levels between Brawly and Roxanne is so small you’d really have to handicap yourself for this gym to be a challenge.
Wattson
RS:
>Magneton Lv 23
>No held item
>Shock Wave, Supersonic, SonicBoom, Thunder Wave
E:
>Manectric Lv 24
>Held item: Sitrus Berry
>Quick, Attack, Thunder Wave, Shock Wave, Howl
ORAS:
>Magneton Lv 21
>No held item
>Supersonic, Magnet Bomb, Volt Switch
Yes, they not only nerfed his Magneton by 2 levels but just like Roxanne it only has 3 moves. And it’s not just Magneton, all his Pokémon have 3 moves.
You can say what you want about whether gen 3 is difficult or not, but they went way overboard with all the nerfs to the gym leaders.