Mega Venusaur doesn't need the sun. It actually doesn't like the sun because that then negates the neutrality to fire it gets from Thick Fat. Regular Venusaur likes the sun so it can get a Chlorophyll speed boost. So use Mega Venusaur on your team.
Somthing that resists/is immune to Ground would be good for the last slot, since Ground is a common attacking type and you are gonna have 2 pokes that are weak to it. Couple of things to consider are Gengar and Togekiss:
-Gengar w/Black Sludge or Life Orb
Nature: Timid
Ability: Levitate
EVs: 252 Sp.Attack, 252 Speed, 4 HP
Moves: Shadow Ball, Focus Blast, Sludge Bomb, Destiny Bond
or
-Gengar w/Black Sludge
Nature: Timid
Ability: Levitate
EVs: 252 Sp.Attack, 252 Speed, 4 HP
Moves: Shadow Ball, Substitute, Disable, Focus Blast/Sludge Bomb
-Togekiss w/Leftovers
Nature: Calm
Ability: Serene Grace
EVs: 248 HP, 252 Sp.Defense, 8 Defense
Moves: Thunder Wave, Air Slash, Roost, Dazzling Gleam/Aura Sphere/Ice Beam/Flamethrower
Oh, and a set for your Klefki:
-Klekfi w/Light Clay
Nature: Bold
Ability: Prankster
EVs: 252 HP, 248 Defense, 8 Sp.Defense
Moves: Reflect, Light Screen, Spikes, Foul Play
Thank you so much. This is very helpful! Now all that is left for me to do is breeding and training them :).