I hate being mainstream, but...
Charizard @ Charizardite X
Ability: Blaze -> Tough Claws
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Dragon Dance
- Flare Blitz
- Dragon Claw
- Roost
Mega Charizard X eats this core for breakfast, and with a side of omelette. Thanks to its immunity to Burns and excellent typing, it can shrugs off any attack thrown by the trio and retaliate with powerful Tough Clawed Dragon Claws.
Vs. Talonflame
0 Atk Talonflame Brave Bird vs. 0 HP / 4 Def Mega Charizard X: 100-118 (33.6 - 39.7%) -- guaranteed 3HKO
252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 248 HP / 0 Def Talonflame: 229-271 (63.7 - 75.4%) -- guaranteed 2HKO after Leftovers recovery
Mega Charizard X doesn't even need the Dragon Dance boost to put Talonflame in the danger zone. Talonflame can't even attempt to stall with Taunt and Roost, because the dark dragon can deal more damage than what it can heal and it can eventually setup with Dragon Dance once Taunt wears off, sealing the falcon's (and its partners') fate.
+1 252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 248 HP / 0 Def Talonflame: 345-406 (96.1 - 113%) -- 75% chance to OHKO
Vs. Starmie
0 SpA Starmie Scald vs. 0 HP / 0 SpD Mega Charizard X: 99-117 (33.3 - 39.3%) -- guaranteed 3HKO
252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 248 HP / 0 Def Starmie: 199-235 (61.6 - 72.7%) -- guaranteed 2HKO
+1 252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 248 HP / 0 Def Starmie: 297-351 (91.9 - 108.6%) -- 50% chance to OHKO
A situation similar with Talonflame, with the difference that Mega Charizard X would have an even easier time setting up on it, since it can take a Scald effortlessly and strike back.
Vs. Ferrothorn
Uh... do we really need to ask?
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 252 HP / 88+ Def Ferrothorn: 708-832 (201.1 - 236.3%) -- guaranteed OHKO (lol)
Sure, the Iron Barbs and recoil damage sucks, but just use Roost and get ready to continue your brutal assaults against the other two.