Reg I is a restricted format, 2 restricteds, so if you're teambuilding with Meowscarada, Cinderace and Greninja, I'd recommend Reg H or F but Reg H isn't on showdown anymore :(. So if you want to continue with Reg I, I'd recommend not using Koraidon, Groudon or Kyogre because they'll lower your Pokemon's damage. Also, don't mean to be rude or anything, but Greninja, Cinderace and Meowscarada aren't really that good in recent VGC, especially after the nerf to Libero and Protean. Meowscarada was good in Series 1, Cinderace in Smogon and Greninja I don't really know, but I think it is or was good in singles.
However, I did see a niche use for Greninja in Reg H on a Rain team with Loaded Dice for self-proc Water Shurikens into their Archaludon for Stamina boosts to maximise Body Press damage. There are more teams that used Greninja but all of them were on Rain teams, at least in top 128. The only results in top 128 I could find for Cinderace in SV is on 2 teams placed 83rd and 96th: https://labmaus.net/teams/discover?date_range=2023-08-27 to 2026-02-18&pokemon_filter=[{"pokemon":"Cinderace"}]&status=official&placement=128. On the other hand, Meowscarada has seen a fair amount of use in Reg H and I also found a 10th place finish with one in Reg G (single restricted format): https://pokepast.es/2e52c89abbc4a1fc
I agree with sumwun on his comment in saying that the answer will be different for different formats, it will, and there are different meta threats you need to be prepared for. I'd recommend not playing a restricted format if this is your core because they aren't well positioned enough to warrant using them over the stronger Grass-Water-Fire core of Rillaboom, Incineroar and Urshifu-R. They have more offensive and defensive between Grassy Terrain used to override other terrains, Incineroar's everything and Urshifu's unblockable damage.
And it is ok to not be serious about VGC, but if you want to see decent success with these specific 3, Reg F would be the best format because Reg H was removed from showdown :(. I'd recommend not using another Grass-Water-Fire core for the team and instead perhaps a Dragon-Steel-Fairy core or just other Pokemon that work well with your chosen 3. Also I'd recommend changing Meowscarada's item to Focus Sash, because Protean was nerfed so Overgrow Sash Meowscarada has been a lot more successful recently.