A hyper offense team is probably what you're looking for. And (Mega) Metagross is a good place to start since it's a pretty good poke. If you're in need of a set for it, there's this:
Metagross @ Metagrossite
EVs: 252 Atk/252 Spe/4 HP
Ability: Clear Body----->Tough Claws
Jolly Nature
-Meteor Mash/Iron Head
-Zen Headbutt
-Hammer Arm/Earthquake
-Ice Punch/Grass Knot/Thunder Punch/Bullet Punch
The last slot can vary depending on the rest of the team, since those moves are meant for more specific Pokemon. You cna also use an Agility set to threaten more hyper offensive teams at the cost of a coverage move.
Mega Metagross hates getting statused though, so you need an offensive Pokemon to absorb that kind of stuff. First things that come to mind are AV Guts Conkeldurr and Poison Heal Gliscor. The former becomes rather dangerous when statused, while the latter is immune to paralysis and wants to get poisoned (be careful around burns/sleep though, it needs to already be poisoned to absorb those without crippling itself). Conk handles Dark types better than Metagross does (especially if you forgo Hammer Arm), while Gliscor is immune to its Ground weakness.
Conkeldurr @ Assault Vest
EVs: 252 Atk/232 SpD/ 20 Def
Ability: Guts
Adamant Nature
-Drain Punch
-Mach Punch
-Knock Off
-Ice Punch/Poison Jab/Stone Edge
Mega Metagross also wrecks most Fairy and Psychic types, stuff Conkeldurr hates.
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 244 HP/92 SpD/72 Spe
Careful Nature
-Swords Dance
-Earthquake
-Knock Off
-Roost
This is a pretty nice Gliscor set, combines bulk and power and isn't too slow, and tends to threaten more balanced teams iirc.
Pick one of the two pokes though, not both Gliscor and Conkeldurr.
I've also heard that Keldeo also works together nicely with Mega Metagross.
As for Talonflame and Greninja, we'll start with Talonflame since it's easier to deal with. Basically any not frail Rock type that doesn't have a secondary type that's Flying or Fire weak will give Talonflame a hard time. So if you wanna be a little more....."hipster", there's always like Rhyperior, Tyrantrum, Gigalith, Barbaracle, Omastar, Golem, Carracosta, etc, take your pick. Having a Rock type also means you'll have access to Stealth Rock, which is needed in 6v6 but not 3v3.
Greninja is much harder to deal with since it's very fast, hits kinda hard thanks to Protean, and has plenty of coverage options and the only move all Greninjas have in common is Ice Beam so you kinda have to play a guessing game on what it's carrying. Not sure what to do about that one.
So where do you wanna go from here?