Salamence @ Choice Band
Nature: Adamant
Ability: Intimidate
EVs: 255 Atk / 255 Spe
IVs: 20+ Atk / 20+ Spe
- Earthquake (TM26)
- Rock Slide (Tutor)
- Aerial Ace (TM40)
- Brick Break (TM31)
This is the lead, most of the time you should click the Aerial Ace or Earthquake buttons, occasionally Rock Slide for something like Gyarados. Brick Break is for Slaking and Walrein. But Brick Break and Rock Slide are *mostly* filler. 
Earthquake is more powerful than Aerial Ace, even with Flying STAB, so unless the target is Evasion spamming, weak to Flying or immune/resistant to Ground, pick Earthquake. 
Latios, Latias, Starmie and some other water types usually are safe enough to switch Salamence out to bring in Registeel. 
The slot 3 Pokémon can be used as a free switch to change the choice locked move when the enemies send out something immune to Earthquake or heavily resistant to Aerial Ace.
The 255/255 EV spread was because laziness, the extra stat point won’t save you from ice beam.

Registeel @ Leftovers
Nature: Bold or Impish
Ability: Clear Body
EVs: 252 HP / 100 Def / 152 SpD / 6 Spe
IVs: 15+ HP / 15+ Def / 15+ SpD 
- Toxic (TM06)
- Seismic Toss (Tutor)
- Amnesia (Level up/Relearn)
- Iron Defense (Level up/Relearn)
Registeel here can Toxic stall most Pokémon with +6 to Defence and Special Defence. Using some PP Ups on Registeel is recommended, but you can probably manage without. 
Toxic is its main weapon and Seismic Toss for decent damage when Toxic isn’t an option, Steel types for example.
Metal Claw is also an option if you can’t get Seismic Toss, it has very high PP so it’s also alright for PP stalling but it’s definitely not ideal. You would also swap to Impish when running Metal Claw. 
Usually you’ll want to raise the more relevant defensive stat first or use Toxic first or second turn. For example, Iron Defence vs a physical attacker or Amnesia vs a special attacker. 
Fighting types like Machamp, Breloom and Hariyama can be problematic, it is best to take them out with Salamence's Aerial Ace when possible. Registeel at +6 Defence can still beat them, but criticals can still happen, so it is still risky.
Swap Defence and Special Defence EVs if using Calm or Careful nature. The higher defensive stat EVs mostly bring the lower stat closer to the higher stat. The initial EV spread was done lazily, but has never needed to be adjusted. 
Do not setup in front of One Hit KO move users, unless absolutely necessary. Sometimes they don’t miss. 

Latios @ Lum Berry
Nature: Modest
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
IVs: 20+ SpA / 20+ Spe
- Dragon Claw (TM02)
- Psychic (TM29/Level up/Relearn)
- Thunderbolt (TM24)
- Ice Beam (TM13)
Latios is my planned 3rd slot Pokémon when I’m not bringing a ribbon master, but I never actually use it or bring it. Calm Mind is a good option too, drop Dragon Claw or Ice Beam if you use it. The Ruby/Sapphire roaming Latios/Latias have four IVs guaranteed to be zero, if you must use one of those, go Timid for Speed. 
Latias, Starmie, Gengar, Alakazam are all fine replacements. Fast and decent special coverage. Timid or Modest are fine for all of them. 
This is the one you drop for the Ribbon Master. 

Walrein @ Quick Claw
Nature: Evil
Ability: Streak Ender
EVs: Too Many
- Sheer Cold
- Fissure
This is not a team member, this is a warning, destroy Walrein on sight. There’s no Sturdy or Explosion on the team, so Salamence and Registeel can easily be taken down by Sheer Cold *if it hits*. It’s rarely worth the risk of setting up in front of Walrein, just take it down fast with Brick Break and Seismic Toss. Rock Slide's Flinch chance can be good for Walrein, but the Accuracy makes it riskier than just using Brick Break. 
There are other One Hit KO move users that can be annoying too, like Seaking and Rhydon. It is best to take them down quickly when you can, don’t setup in front of them. Counter can also be a problem, usually Toxic stall is preferable for Counter users, but sometimes you have to attack anyway.