Well first, let's see what type weaknesses this core might have.
Gengar is weak to Psychic, Ghost, and Dark.
Tyranitar is weak to Water, Grass, Fighting, Bug, and Steel.
Magnezone is weak to Fire, Fighting, and Ground.
There are very few common weaknesses, so there are no teams members needed to remedy that. Let's look at their roles:
Gengar: Fast Special Sweeper
Tyranitar: Slow Bulky Physical Attacker / Setup Sweeper
Magnezone: Slow Bulky Special Attacker
So, you're gonna want to Attack from both sides of the spectrum, so we'll need a fast physical sweeper. In addition, Stealth Rock support is always very nice, and clerics are almost necessary with an offensive team like this.
For the fast physical sweeper, I suggest Mega Gallade. Great bulk, high Attack, and awesome Speed. Here's the set I would run:
Gallade @ Galladite
Ability: Justified -> Inner Focus
EVs: 252 Atk / 4 Def / 252 Spe
Gallade adds a Flying, Ghost, and Fairy weakness to the team. Flying and Ghost are covered by T-tar, and Fairy is covered by Magnezone. In addition, Gallade can switch into Knock Offs meant for Gengar, snatch an Attack boost, and destroy.
Now, I can see Landorus as a very good partner for this team. You can VoltTurn with Magnezone, and can serve as a secondary switch-in on Ground moves meant for Magnezone. Landorus adds a Water and Ice weakness. His VoltTurn partner Magnezone resists Ice, and can take care of Water. Here's the set I would run:
Landorus @ Leftovers
EVs: 252 HP / 248 SpD / 8 Spe
I went a specially defensive route with this one, as Magnezone has high physical Defense. Intimidate "increases" his Defense, so SpD investment is good too.
Last but not least, a cleric. Someone who can save your Pokemon when their HP is low, and restore status if necessary. I think the one that would fit the bill best is Latias.
Latias is weak to Bug, Ice, Ghost, Dragon, Dark, and Fairy. Bug, Ice, Dragon, and Fairy are covered by Magnezone, and Ghost and Dark are covered by Tyranitar. Here's the set I would use:
Latias @ Leftovers
EVs: 252 HP / 252 Def / 4 SpD
Latias Defogs away opposing rocks, sets up Wish to heal you or your switch-ins, Healing Wish is a last-ditch effort to sacrifice yourself and fully heal one of your Pokemon.
Overall, I think this is a well-rounded team, and would work well in competitive battling. Good luck, and tell me how it goes!