The Ryu Alpha Armor Set is part of the new challenge quest called “The Awakened Satsui No Hado” where you fight Nergigante. By completing one of them you are rewarded with SFV III Tickets that unlock the Ryu Alpha Set.
The full Ryu Alpha armor set requires 7 SFV III Tickets
The difference between the 3 quests is the weapon and armor loadout for the arena, but rewards are the same.
If you have trouble finding someone to complete these arena quests with, make sure to join an online session with the preference Arena.