I could use some help with team-building

My main problem when making teams isn't the individual Pokemon themselves. It's team synergy. I have issues creating a team that actually works well together.

So what I was thinking was this: Singles battles in the Battle Spot only let you use 3 Pokemon in a match, not your whole team of 6. I would like to have 2 groups of 3 Pokemon that work well with each other, so I don't have to guess as much as to what Pokemon I should use against the other person's team (yes, team preview helps, but still).

So I'll start off with 1 Pokemon for each half-team/core. I want you guys to tell me which 2 Pokemon would work well with them. You don't have to be terribly detailed, I can work out the individual sets myself (probably).

-Half-Team 1: Greninja
1) Greninja w/Life Orb
Ability: Protean
Nature: Hasty
EVs: 252 Sp.Attack, 252 Speed
Moves: Hydro Pump, Ice Beam, Dark Pulse, Spikes/U-Turn

2)????
3)?????

-Half-Team 2: Mega Lucario
1) (Mega) Lucario w/Lucarionite
Ability: Adaptibility
Nature: Hasty
EVs: 252 Sp.Attack, 252 Speed
Moves: Aura Sphere, Dark Pulse, Flash Cannon, Extreme Speed

2)?????
3)????

Remember, this is for Battle Spot battles, not Pokemon Showdown battles, and Greninja and Lucario will probably not be fighting together.

For Lucario's team, you already have perfect coverage with the moves you have, so to patch up the team defensively, you'll just need to cover Lucario's weaknesses. Water/Flying types are good examples as they patch up all three of Lucario's weaknesses, and a Ground type because it can switch in to resist the nasty Electric-type that Water/Flying types are weak to.

As for Greninja's team, any Flying type would be necessary offensively, as the Water, Dark and Flying moves have perfect coverage altogether. It doubles up on your Electric-type weakness however, so a Ground-type would be the ideal third Pokemon as it will help synergize your team defensively. Hope this helps.

Thank you for your response, Boilurn. It's funny that you mention a Water/Flying type. I have this nice Gyarados all ready to go...but its on my Black2. Waiting for the PokeBank is getting hard.

For Lucario's half team, I'd rather use a physical set on it, as special sets function best with Nasty Plot (which Riolu can only learn) or Agility (egg move):
-Sword Dance
-Close Combat
-Extreme Speed
-Earthquake/Crunch/Shadow Claw
Nature: (whatever you have)
EVs: 4 HP/252 Atk/252 Spe
Item: Lucarionite

Teammates that can check/counter Aegislash and Talonflame are important. Wash Rotom takes care of Talonflame, while Garchomp counters/checks Aegislash well:
Wash Rotom:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Rest
Nature: Bold
EVs: 252 HP/252 Def/4 SAtk
Item: Chesto Berry

or
-Discharge
-Volt Switch
-Hydro Pump
-Hidden Power (Ice)
Nature: Modest
EVs: 232 HP/56 SAtk/220 Spe
Item: Assault Vest

or
-Volt Switch
-Hydro Pump
-Hidden Power (Ice)
-Trick/Thunderbolt
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Scarf/Choice Specs

Garchomp:
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Stone Edge/Fire Fang
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Lum Berry/Yache Berry/Haban Berry

or
-Outrage
-Earthquake
-Stone Edge/Fire Fang/Fire Blast
-Dragon Claw/Dual Chop
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band/Choice Scarf

For Greninja, something that can take out special walls is best, like Aegislash:
-Swords Dance
-Autotomize
-Shadow Claw
-Iron Head/Sacred Sword
Nature: Adamant
EVs: 88 HP/252 Atk/168 Spe
Item: Life Orb

or
-Shadow Ball
-Shadow Sneak
-Iron Head
-Sacred Sword
Nature: Quiet/Brave
EVs: 4 HP/252 Atk/252 SAtk
Item: Spooky Plate/Spell Tag/Life Orb

or
-Swords Dance
-King's Shield/Shadow Claw
-Shadow Sneak
-Iron Head/Sacred Sword
Nature: Adamant
EVs: 252 HP/252 Atk/4 Spe
Item: Leftovers/Life Orb

or
-Autotomize
-Shadow Ball
-Hidden Power (Ice)
-Iron Head/Sacred Sword
Nature: Rash
EVs: 88 Atk/252 SAtk/168 Spe
Item: Life Orb

Dragon Pokemon pair well with Aegislash too, like Goodra:
-Draco Meteor
-Fire Blast/Flamethrower
-Thunderbolt/Sludge Bomb
-Focus Blast/Dragon Tail/Power Whip
Nature: Modest/Quiet
EVs: 252 HP/4 Def/252 SAtk
Item: Choice Specs/Assault Vest
Ability: Gooey/Sap Sipper

or
-Dragon Pulse
-Flamethrower/Fire Blast
-Thunderbolt
-Toxic/Dragon Tail
Nature: Calm/Sassy
EVs: 252 HP/4 SAtk/252 SDef
Item: Leftovers
Ability: Gooey/Sap Sipper

(just an example; there are Dragon Pokemon that are as good as Goodra out there)

Thank you very much, Platinum Dude. Nowe to start EV training and such...