Latios @ Twisted Spoon
Ability: Levitate
Level: 50
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Psychic
- Ice Beam 
- Thunderbolt
- Protect
Twistedspoon because the power bonus picks up a lot of extra KOs and supercharges your stab move. Hax aren't nearly as big of an issue in doubles to warrant a lum berry, and you need the extra power since you can't fit calm mind on the moveset over protect. Protect is 100% mandatory to dodge explosions and protect Latios on bad turns and sometimes during switchins. Faster timid latios are preferred but modest can still work fine too. Ice beam is necessary over Dragon Claw in doubles since the coverage is super useful and dragon claw doesn't pick up enough KOs that ice beam doesn't to be worth the massive loss of coverage and the consistent dragon killing of ice beam. Psychic obliterates almost every fighting type in one hit, especially blazikens and breloom. 😉 Make sure latios destroys any gengars it sees so Metagross and Snorlax can freely explode at their leisure. 

Metagross @ Choice Band
Ability: Clear Body
Level: 50
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Explosion
- Meteor Mash
- Earthquake
- Shadow Ball
The standard Choice Band Metagross move set, but with max attack and max speed. You NEED to be max speed for doubles, you need your explosion to go off as quickly as possible if you blow up on turn 1. If you see a fire type that outspeeds metagross (i.e., anything faster than Flareon) switch it out to snorlax IMMEDIATELY or you will 100% get overheated or fire-blasted and instantly die. Blow up on any threatening bulky waters, especially on ones with OHKO moves like Walrein, Dewgong, Wailord, Whiscash, Lapras, etc., and ESPECIALLY if you see two of them at once. Also, blow up on any enemy snorlax you see, you want to get rid of them immediately, as they are some of the most dangerous sets in the tower. Choice Band Explosion kills almost every single mon in the game, and most frailer steel types. Registeel will live on like 1-20% health, Scizor will die, Steelix will live, Most Metagrosses will die (thought they will also just flat die to CB EQ), and Aggron takes like 40-50% due to its quad resistance. Most matches, you will be spamming EQ to kill the majority of mons, with Latios helping Meta to weaken any mons not weak to EQ (or mons that don't naturally die to it anyways) so that EQ can kill both enemy mons on turn 1, thus keeping your team's health perfectly intact. 

Snorlax @ Silk Scarf
Ability: Thick Fat
Level: 50
EVs: 252 HP / 252 Atk / 4 Def
Adamant Nature
- Self-Destruct
- Body Slam
- Earthquake
- Shadow Ball
Self Destruct is a one time tutor move for Lax in XD, and yes it's pretty . Possibly the most broken pokemon in Gen 3 Doubles, Snorlax is an absolute MONSTER. There is almost nothing besides OHKO moves and other explosions that can kill this thing in one hit. Armed with the most powerful explosion in the game, Snorlax can be your second slower bomb to wipe out what remains of the enemy team. More importantly though, you can use it as a second metagross that can change its moves (which is a BIG deal) since it has very similar coverage, way more bulk, and resists both fire and ice with thick fat, allowing it to switch in for either latios or metagross, making Snorlax their perfect partner. A lot of time you won't need to explode and can just use Snorlax normally, which is perfectly time and generally safer unless you know you can win otherwise. You can also delay exploding until Turn 3 if you want so that Latios's protect is back up to 100% success rate after you used it turn 1. Or you could do the Yolo Maneuver by just blowing up on turn 2 and wiping out your own latios to ensure the battle ends in a 1-0, assuming no protect or evasion shenanigans occur.

Gengar @ Lum Berry
Ability: Levitate 
Level: 50
EVs: 252 SpA / 6 SpD / 252 Spe 
Timid Nature
- Psychic
- Thunderbolt
- Ice Punch
- Protect
Most games Gengar won't have to do anything, but it's not because he's useless, it's because he enables the other 3 members to win the vast majority of matches very consistently. He allows the player to be far more aggressive in their explosion plays, since there is no longer any risk of latios getting doubled into and dying, forcing a non ghost mon to come in and immediately die to Snorlax's self destruct on turn 2. Gengar gives you the protection of a ghost to avoid explosions while simultaneously being a ghost type Latios with the same coverage and speed, while possessing crucial immunities to 3 incredibly prevalent types. Lum berry is the item of choice to soak any late game bullshit status moves. Psychic lets it kill other ghosts, Tbolt for coverage, and ice punch to kill dragons and the Latis with. Protect is there to keep gengar safe for an extra turn while lax or its partner cleans up any threats to it, and because the AI will love to target gengar cause it's the easiest thing on the team to kill given its frailty. Gengar is also the mon who gets replaced on the team for your ribbon master.
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Always lead Latios + Metagross. Most games you'll be spamming EQ with Meta while Latios snipes priority targets with its coverage, or puts a bulkier mon into EQ range (that would normally survive) for Meta to kill alongside a hopefully frailer or ground weak enemy partner. If there are two incredibly bulky targets such as two bulky waters, a scizor, a cradily, OHKO users, etc., you can just protect with Latios and explode with meta turn 1 to instantly delete them. You can do the same thing turn 2 with just leaving latios to die on the field to wipe out the last two mons. You can also OHKO something with latios and blow up turn 1 to trade Latios and Metagross for 3 enemy mons (assuming they have no ghosts and brightpowder doesn't activate). This leaves Snorlax and Gengar to kill 1 final enemy mon, and there's almost nothing that can take them both out. 
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Mons that live explosion: All ghosts, Aggron (always) Steelix, Registeel (gets put in red), Regirock, other super bulky steels and rocks. Anything that resists explosion and is as (or less) bulky than a scizor will die to explosion.