I think that you should try to get one mega evolution instead of 3-4 possible ones, and build your team around that. So, depending on what you want to start out with, and what tier you want to play in, it all depends. I think you already have good roles for your Pokémon, I would maybe use Gengar as a special sweeper. Ninetales could also play a part in being a special sweeper. Here are some evaluations on the possible mega evolutions for your team.
Lucario: Mega Lucario is in the Ubers tier, and is one of the best fighting type possible mixed sweepers. Although it is quite frail, it has both a threatening Attack and Special Attack stat. You do have to watch out for ground and fire types, especially Primal Groudon in the Ubers tier. You can try out Mega Lucario if you need a powerful mixed sweeping mega evolution.
Garchomp: Mega Garchomp is in the OU tier. Although it looses speed when it mega evolves, it gains in attack power. Watch out for certain OU Pokémon, like Keldeo and Manaphy, both having access to ice type moves. I would use Mega Garchomp if you want a strong attacker.
Gengar: Mega Gengar is in the Ubers tier, like Mega Lucario. Mega Gengar is a powerful and fast special sweeper, so if you want a reliable fast sweeper, Mega Gengar could be an option. Although, Mega Gengar is weak to other Ubers ghost types and potential ground types as well.
Those are my brief overviews over the three mega evolutions that you may be trying to decide from, hopefully this helps to narrow it down a little bit. Personally, I would choose one mega and build my team around that core, and try to build around that mega, seeing what it would need to cover its weaknesses and such. Good luck!
