Why couldn't i use this card as a substitute for ancient gear golem in order to summon ultimate ancient gear golem? i had this card on the field + 2 ancient gear monsters, and i couldn't activate the effect.
Also, you can only use Earth Hex as the AGG replacement.
By any chance, is any of the other two AG monsters an AGG? The game might treat it as the main AGG material instead of one of the 2 AG monsters, and thus Earth Hex is not replacing AGG, as a result, you cannot summon UAGG.
You can only use a Fusion Substitute over a named material. It's not easy to prove in this particular case since you can only use 1 Sub and it substitutes over the Ancient Gear Golem (and AGG can be one of the "Ancient Gear" materials), but you can't use it as one for a monster like Pair Cycroid.