Mystic Forge Error?
The stone takes the place of a fourth item of equal rarity, with level equivalent to the average level of the first three items. The replaced item will be either a generic armor, trinket or weapon, depending on the other 3 items used, but will not have a specific subtype (e.g. it will be “a weapon”, not “an axe”)
The MysticForge stones are the same as a random item.
From the same wiki page
Three Level 75 exotic axes + 1 stone = same as four random Level 75 exotic weapons; result will be a Level 80 random exotic weapon.
Be careful what you ask for
ANet may give it to you.
ANet may give it to you.