Hoenn Starter Trio
The Hoenn Starter Mega stones have several similar mechanics. They start with a small story starting with Amy/Wally/Prof Cosmo somewhere in Hoenn (after hoenn championship) leading up to an item you need to craft from 10 high IV starters. After which you unlock the main character from the quest as Champion you can battle daily (or weekly not sure, will double check next time i have time).
The Mega Stone will be an incredibly rare one time loot drop from these champions if you won in hard.
Blazikenite *Blazikenite icon*
May has issues with her Blaziken atop Mt. Chimney.
Not entirely sure but I think she immediately requests you to acquire a Red Flute to calm down Blaziken.
Get Red Flute for May
Help May by obtaining a Red Flute to calm her Blaziken. Visit the Flute Maker near Route 113 to craft the flute using claws from high IV Torchics caught in the Safari. This special item is crucial for May's journey and her partnership with Blaziken.
You'll need 10 high IV (120 or up, will be changed to 100 or up if not already changed) Torchics, the NPC taking the torchics will not return them. The torchics will be lost forever.
After collecting 10, you can exchange those for a Red Flute near the very same NPC exchanging the torchics (Route 113 as said in quest tracker).
Calm Down Mega Blaziken
Use the newly acquired Red Flute to calm down May's Mega Blaziken at Mt. Chimney Peak.
Faint the Blaziken in battle (interact with him & win battle, he's uncatchable).
Speak with May
After calming Blaziken, May has some things to say. Speak to her before you leave Mt. Chimney Peak.
After fainting the blaziken, you go back to Amy to learn where she will be going.
Her champion can now be found in the Lilycove Mall.
Sceptilite *Sceptilite icon*
Prof. Cosmo has issues with his Sceptilite south of the Weather Institute.
Not entirely sure but I think he immediately requests you to acquire a Lighting Condensor to calm down Sceptile.
Collect Treecko Scales
Professor Cozmo and the Weather Institute need Treecko Scales to craft a lightning condenser to calm the rampaging Sceptile. These scales are key to stabilizing the weather and calming Sceptile.
You'll need 10 high IV (120 or up, will be changed to 100 or up if not already changed) Treecko's, the NPC taking the treecko's will not return them. The Treecko's will be lost forever.
After collecting 10 Treecko Scales, you can exchange those for a Lighting Condensor near the very same NPC exchanging the Treecko's (weather institute 2nd floor).
Calm Down Mega Sceptile
Use the lightning condenser to calm down the rampaging Mega Sceptile. The device, powered by Treecko Scales, should stabilize the weather and soothe Sceptile
Whelp, do what the quest tracker says.
Speak with Professor Cozmo
After calming Sceptile, Professor Cozmo wants to thank you. Speak to him before you leave.
Uniformity 
Champion Cozmo can be found in his house in Fallarbor.
Swampertite *swampertite icon*
Swampert is a bit different from the other 2, in the way that his story is a bit longer.
Wally can be found just outside the Hoenn E4 building, near a mega swampert.
After talking to both Wally & the Swampert, the swampert ran off.
Speak with Wally
After your encounter, Mega Swampert has fled. Consult with Wally to discuss how to proceed and calm Mega Swampert down.
Well, yeah.
Find Mega Swampert in Sootopolis
Wally believes Mega Swampert may have returned to Sootopolis, its natural habitat. Search for Swampert in Sootopolis to see if it has sought refuge there.
Sootopolis isn't that big, but he'll be in the south western area.
Report Back to Wally
After encountering Mega Swampert in Sootopolis, it fled once more. Report back to Wally outside the Hoenn League to discuss the situation and plan your next move.
Again, pretty straight forward in quest tracker.
Prepare the Mystic Potion
Wally has asked for your help in calming down Mega-Swampert. Meet the scientist in Sootopolis to prepare the Mystic Potion using star-shaped gills from Mudkips.
And here we have the Swampertite's equivalent of Red Flute/Lighting Condensor.
You'll need 10 high IV (120 or up, will be changed to 100 or up if not already changed) Mudkips, the NPC taking the Mudkips will not return them. The Mudkips will be lost forever.
After collecting 10 Star-Shaped Gills, you can exchange those for a Mystic Potion near the very same NPC exchanging the Mudkips (Sootopolis building right of the pokecenter).
Take the Mystic Potion to Wally
Wally needs the Mystic Potion to calm down Mega-Swampert. Deliver the potion to him outside the Hoenn League.
Follow Wally to Calm Mega-Swampert
With the Mystic Potion in hand, follow Wally to the Seafloor Cavern to help calm down Mega-Swampert.
Again pretty much everything is said in the quest tracker, but the guide wouldn't be complete without steps. (Seafloor Cavern (room 5))
Champion Wally can be found in Wally's Room (last map of the Hoenn champions road).
