Well first off, I got them off GTS. I wasn't gonna waste time training up my own guys.
Second off, they're mixed attackers to exploit more weaknesses.
Third off, Only Tapu Koko had its Z-Crystal (Tapunium Z)
Fourth off, Thunder worked really well thanks to both its STAB and Beast Boost and the 2 Bug attacks on Pheromosa are also for STAB and to counter its Psychic-Type weakness.
Fifth off, Nature's Madness was for the aforementioned Z-Crystal.

That still doesn't tell me if all 3 were EV trained. You can check that by pressing Y during the summary screen
Then I'm afraid you don't really understand it as to exploit more weaknesses, the better thing to do is type coverage
Wouldn't an Electric Z-Move be better? The Tapu's Z-Move always does 75% of what the enemy has and Tapu Koko's the kind of mon that would rather OHKO as it's not too bulky but still fairly strong
Almost every Psychic type has pretty low Defence so Lunge should be good enough
Again, I don't see the point of having this move as it's got a fixed amount of HP which doesn't really complement Koko's stats

Tapu Koko
-Get rid of Nature's Madness, the only Tapu it's good on is Fini
-Replace Aerial Ace with Brave Bird
-Give it a Life Orb(Found in Resolution Cave where Guzzlord is found)
-Go to the Daycare and check if it has Hidden Power Grass or Ice, it it has either, teach it Hidden Power over Nature's Madness.
-Give it Thunderbolt over Discharge

Pheromosa
-Give it a Focus Sash(From a Hula Girl in Poni Wilds on the beach)
-Ice Beam is fine for a mixed set, but you already have Koko basically mixed because of Brave Bird.
-Replace Brick Break with High Jump Kick(You can even give it the Fighting Z-Crystal if you want to avoid missing for one turn)

Xurkitree
-I would replace this with something else. You already have Koko for Electric coverage
-If you can't replace it, teach it Energy Ball(Found in a cave accessed from Lush Jungle)
-Give it the Psychic Z-Crystal and teach it Hypnosis. Z-Hypnosis will raise your Speed by 1 stage and will allow you to set up a Tail Glow allowing you to most likely wipe out the entirety of your opponents team
-Teach it Tail Glow
-Replace Thunder with Thunderbolt

Got the Stones. I was actually able to find someone willing to trade.

I still appreciate all the help you guys gave. I have been wanting to play competitively so maybe I'll use your tips for VGC.

I don't know if this happened to anyone else here but it's happening to me now, the night before I was on the GTS attempting to chain trade for a Shiny Dhelmise and I ended up getting softbanned. More than 24 hours later, still unable to trade.

How long before I sue Nintendo for my pain and suffering?

Probably because you hacked the game, it loses the possibility to connect to the internet, same with genning pokemon every now and then. It's why some tubers like HoodlumScrafty and DuncanKneeDeep have multiple version of the game and multiple 3DSes each

I didn't hack at all. I was legit trading when this happened.

Editing Pokemon is fine as long as you make them look realistic and you shouldn't get banned for that.

However, if you edit items or anything about your trainer(Money amount, name etc) that will get you banned from online, especially if you have that game connected with the Global Link.

Edit: If you didn't hack like you claim, you probably got a badly hacked Pokemon someone put up for trade on GTS.

I don't know, all I know is it was a Mewtwo I was trying to trade for before I got the communication error that led to this softban.

Finally able to trade again!