I'm not sure what you mean by picking 3 then 6 but I am going to list the pokenon I have experience with and why they are good.
Alakazam - Magic Gaurd let's Alakazam abuse 2 items, Focus Sash and Life Orb. Alakazam has a large move pool with stellar Special Attack and good speed. It doesn't take damage from things like Status, Weather, and Hazards, meaning you have to attack it if you want to break it's sash, or if you want to run Life Orb you don't take recoil from it, so you get a free 30% power boost to all of your moves, making for a good sweeper. Just watch out for any sort if Priority bar Mach Punch, as Alakazam is frail.
Jolteon - Joltreon works very well in Rain, abusing STAB Thunder with his insane speed and decent SAtk. The best set Jolteon can run is a Choice Specs set, which hits incredibly hard and is still fast enough to effectively revenge kill. Thunderbolt also works outside of rain and Volt Switch hits surprisingly hard. Just watch out for Dugtrio, and opponents that have priority/are scarfed. I find that people tend to underestimate Jolteon, and that's a mistake as he can devastate unprepared teams.
Gengar - With great SAtk and Spd, access to Substitute and almost perfect coverage in Focus Blast and Shadow Ball, Gengar is a force to be reckoned with. I usually max his SAtk and Spd with Timid Nature, running Shadow Ball, Focus Blast, Substitute or Destiny Bond, and Disable/Pain Split/Taunt. As you can see, despite being extremely frail, Gengar is very versatile and has a plethora of options.
Salamence - Choice Scarf + STAB Outrage + Moxie = What is quite possibly on of OU's best late game sweepers. Basically you just clear out all the Steel types and spam Outrage, and with his decent bulk, he only fears Ice Shard and other faster scarfers such as Terrakion. One thing that kind of tears him down a little bit is SR, but either way if you can secure an initial KO you will be severely punishing just about anything that isn't killed. Note that Outrage can and will deal lots of recoil from confusion if you have acquired Mutiple Moxie boosts, so it is usually foolish to send him out early in the game.
Zoroark - I freaking love Zoroark. It is so tricky and has so many options. Usually you want to disguise it as something that will bring in physical walls, where you can destroy them with Special moves. The best sets one can run with Zoroark are a Choice Specs set or a Nasty Plot set, taking advantage of his great SAtk and good speed. A Choice Band or Swords Dance set would also be possible but Zoroark has better options on the special side including Night Daze, Flamethrower, Focus Blast, etc. Zoroark is frail and will fall to priority, so he relies heavily on Illusion to keep him alive by disguising itself as something that won't be killed by priority.
Meinshao - Meinshao is an animal. With a huge attack and speed stat, Meinshao has access to one if the best abilities in the game, Regenerator. Regenerator allows Meinshao to abuse a Life Orb, as he can take most if the edge off of LO recoil by merely switching out. Meinshao has has a very good move pool with lots of powerful options such as Hi Jump Kick, Stone Edge, Fake Out, U-turn, Acrobatics, etc.
Volcarona - Volcarona needs Rapid spin support to sweep effectively, but get 2 or more Quiver Dances up and you have basically won. STAB Bug Buzz and Fiery Dance hit hard, and Fiery Dance will often get you a SAtk boost. He can evens use rain with Hurricane, which allows him to work effectively as a sun and rain sweeper, and I use him on Rain teams with Fiery Dance to counter sun and hit Steel types.
A lot of the sweepers you have listed work very well, but these are the ones that I have used and know they work well.
