Need help with my first competitive team!

[Frokage]

Hello PC battle enthusiasts! ^^
I've never battled competitively before, so I'd like to hear your opinions on what I've managed to build so far. Below each pokémon I'll write a short description of the pokémon's main role/task in the team. Oh, and this is for Single Battles!
Thanks!

1. Excadrill @ (?)
Nature: Adamant
Ability: Sand force
252 Attack / 252 Speed / 4 HP

-Earthquake
-(?)
-(?)
-(?)​

Alright, so Excadrill should basically be one of my team's main powerhouses. I surely hope its speed will be enough to outrun most pokémon, but I doubt it. Aside from that, moves to power up attack and such would be good, right?

2. Hydreigon @ (?)
Nature: **CURRENTLY HARDY, BUT I CAN BREED IT FOR OTHERS**
Ability: Levitate
252 Attack / 252 Speed / 4 Sp. Def

-Draco Meteor
-Dragon Pulse
-Dragon Dance
-Shadow Claw
​

Hydreigon's one of my all time favorite Pokémon! The three headed team sweeper! The only problem with this one is that I don't know what Nature would be best to suit its style. At first I went with a fully Sp.Atk moveset, but then considered the regular Attack approach. Any ideas to improve this fella?

3. Blissey @ Leftovers
Nature: Bold
Ability: Natural Cure
252 HP / 252 Defense / 4 Sp. Def

-Toxic
-Soft-Boiled
-Protect
-Minimize (?)*​

*I didn't know up until recently that Minimize is a forbidden move, so I'm looking for something to replace it with.
Alright, so Blissey is my team's main tank. The idea of Blissey is to have it absorb most damage, heal itself and stall some time as it watches her poisoned opponent die slowly.

That is as much as I have at the moment. I still lack three Pokémon, but I have no idea which ones to pick. I've been thinking about using my Frokage, but there are undoubtably better Pokémon to pick. I could probably use one more tank, and two more powerhouses. Any ideas guys?

Thank you all! :)

 

[PlatinumDude]

You may need to do some more research on what Pokemon can/can't learn.

Outside of Sandstorm, Mold Breaker is Excadrill's preferred ability to let it hit Levitate users with Earthquake:
-Earthquake
-Rock Slide
-Rapid Spin
-Shadow Claw/Swords Dance
Nature: Adamant/Jolly
EVs: 252 Atk/4 SDef/252 Spe
Item: Assault Vest/Life Orb/Air Balloon
Ability: Mold Breaker

Hydreigon can't learn Shadow Claw or Dragon Dance. The 4x weakness to Fairy severely hinders it, so it needs Flash Cannon to even get past them:
-Draco Meteor
-Dark Pulse
-Flash Cannon
-Roost/Fire Blast/Focus Blast
Nature: Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb

Blissey is best known for being a cleric (healer of status conditions) for the team:
-Aromatherapy
-Softboiled
-Seismic Toss/Flamethrower
-Toxic
Nature: Bold
EVs: 4 HP/252 Def/252 SDef
Item: Leftovers
Ability: Natural Cure

You need Fighting resists on the team, because all three of your current Pokemon are weak to them. Examples include Sylveon, Gengar, Scolipede, Gliscor, Aegislash, etc:

Sylveon:
-Wish
-Protect
-Moonblast
-Shadow Ball/Hidden Power (Fire)
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers

or
-Calm Mind
-Moonblast
-Shadow Ball
-Hidden Power (Fire)
Nature: Modest
EVs: 252 HP/252 SAtk/4 Spe
Item: Leftovers

Gengar:
-Shadow Ball
-Sludge Bomb
-Substitute/Protect
-Focus Blast/Hidden Power (Fire)
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Black Sludge/Life Orb

or
-Shadow Ball
-Sludge Bomb
-Focus Blast
-Hidden Power (Ice)/Trick
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Scarf

or
-Shadow Ball
-Sludge Bomb/Focus Blast
-Taunt
-Destiny Bond
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Focus Sash

or
-Shadow Ball
-Sludge Bomb
-Focus Blast
-Destiny Bond/Taunt
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Gengarite

Scolipede:
-Spikes
-Swords Dance/Toxic Spikes
-Megahorn
-Baton Pass
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Focus Sash
Ability: Speed Boost

or
-Swords Dance
-Megahorn
-Earthquake
-Rock Slide/Baton Pass
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Ability: Speed Boost

Gliscor:
-Earthquake
-Toxic
-Substitute/Taunt
-Protect/Roost
Nature: Impish
EVs: 244 HP/252 Def/12 Spe
Item: Toxic Orb
Ability: Poison Heal

or
-Earthquake
-U-turn
-Taunt
-Roost/Knock Off
Nature: Impish
EVs: 244 HP/48 Def/216 Spe
Item: Toxic Orb
Ability: Poison Heal

or
-Substitute
-Baton Pass
-Earthquake
-Taunt/Swords Dance/Agility/Rock Polish
Nature: Impish
EVs: 244 HP/48 Def/216 Spe
Item: Toxic Orb
Ability: Poison Heal

Aegislash:
-King's Shield
-Shadow Ball
-Shadow Sneak
-Iron Head/Sacred Sword
Nature: Quiet
EVs: 252 HP/4 Atk/252 SAtk
Item: Leftovers

or
-Swords Dance
-Autotomize
-Shadow Claw
-Iron Head/Sacred Sword
Nature: Adamant
EVs: 88 HP/252 Atk/168 Spe
Item: Leftovers

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

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

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

This team is also somewhat vulnerable to bulky Waters, so something like Trevenant or Gourgeist also provide team synergy:

Trevenant:
-Wood Hammer/Horn Leech
-Earthquake/Rock Slide
-Will-o-Wisp
-Rest
Nature: Impish
EVs: 252 HP/176 Def/80 Spe
Item: Leftovers/Lum Berry
Ability: Natural Cure/Harvest

or
-Substitute
-Leech Seed
-Will-o-Wisp
-Phantom Force
Nature: Impish
EVs: 252 HP/176 Def/80 Spe
Item: Sitrus Berry
Ability: Harvest

Gourgeist (Super Size):
-Will-o-Wisp
-Leech Seed
-Protect/Pain Split
-Seed Bomb/Shadow Sneak
Nature: Impish
EVs: 252 HP/4 Atk/252 Def
Item: Leftovers
Ability: Frisk

 

[Frokage]

Thank you so much for the immense help, mate! And I truly had no idea about the movepools these Pokémon have. :/ Thanks again!