Chesnaught@Leftovers
Bulletproof
Impish 252 HP / 4 At / 252 Df
~ Spikes
~ Leech Seed
~ Spikey Shield/Substitute/Roar
~ Hammer Arm/Seed Bomb
Tyranitar@Choice Band/Scarf/Tyrantite
Sand Stream
Brave/Adamant 252 At / 252 HP/Spd
~ Pursuit
~ Ice Beam/Ice Punch
~ Fire Blast/Fire Punch
~ Stone Edge/Stealth Rock
Aegislash@Balloon/Leftovers
Stance Change
Brave 252 HP / 252 At / 4 SpA
~ Kings Shield
~ Sacred Sword
~ Shadow Sneak/Shadow Ball
~ Flash Cannon/Iron Head/Swords Dance
Here is a trio epitomizing bulky offense, or in this case more semi-stall. None of these three are left wanting for power or durability. Across all three no type is not covered in some fashion. Tyranitar and Chesnaught form a double hazard duo with lost of accessory means of passive damage to force switches like Leech Seed and Sandstorm. Across the two only Fairy is a common weakness, and Aegislash makes a notable third part of this core taking Fairy attacks as well as Fighting, Dragon and Ice on top of that, as well as performing as a spinblacker all in one. A banded Tyranitar is great for some real power and the mixed attacking helps a lot. You're really going to want Ice Beam for Gliscor who could be a real thorn in your side otherwisem and Fire Blast is great for getting rid of Skarmory and Ferrothorn early. Pursuit is great for taking care of Defog Latias. Scarf on the other hand helps with some of your speed issues and helps land some surprise KOs. Spikey Shield and Kings Shield together yield more turns for free passive damage and Lefties/Leech healing. The one thing that would give these guys trouble though is status, so here's the penultimate 4th.
Florges@Leftovers
Flower Veil
Calm 252 HP / 252 Df / 4 SpD
~ Wish
~ Aromatherapy
~ Protect
~ Light Screen/Moonblast/Toxic
Florges is one crazy special sponge and pure Fairy typing is really something. Florges provides Wish to help your core keep on trucking, further Protect stall, and oh so handy Aromatherapy to remove that pesky debilitating status from everyone. Light Screen in a fourth slot can provide just a few extra turns of safeness or you could use Moonblast so you have something for Taunt. Likewise a Clefable can do all of the above and Magic Guard protects from all passive damage, or you could use a Blissey/Chansey.
Some of the EVs and sets can obviously be optimized, but I think this core is monstrous.