I know that all current Mythics are 1 piece, but bear with me.
The 3 piece set is called "Knowledge is Power"
All three pieces are level 1 only, so they can be worn my anyone.
Wear 1 piece, get a 50% XP boost, 2 increases it to 100% XP, 3 becomes 150%XP.
XP potions and scroll do not stack with it, but Seasonal XP buffs do stack with it.
These pieces have NO armor value, enchantment or Trait, but they can be enchanted and Traited.
Piece 1 "Thinking Cap" Light Head piece
Lore tab: This light-weight metal hat appears to made of some cheap metal like tin, that has been hammered as thin as paper and shaped by hand to fit ones head. A note found with the hat states the it's creator used it to block out the daily distractions of the Mundus to allow them to focus on the task at hand. Scrawled on the back of the note, it goes on to say that hat also blocks out the whispering influences from the Daedric realms.
Piece 2 "Gloves of Practical Application" Medium Hands piece
Lore tab: These cursed glove are believed to have belonged to a farmer that, due to the blood-stained holes in the fingers, literally worked them to the bone in what appears to be some sort of grinding motion.
Piece 3 "Nowitol's Necklace of Wisdom Teeth" Amulet
Lore tab: Legend has it that a rather brutish, but dim-witted (and ironically named) Orc "persuaded" some of Tamriels brightest men and mer to "donate" these teeth in a misguided quest to make himself smarter.
The down side of these should be obvious. No monster set, no 5-5-2 build. This is not a combat set, just farming XP and leveling up.