Okay, I'm going to play.

The most correct approach. 2 APJIAC In the game Heroes 3 (at least in the later versions, I can't vouch for the very first ones, without patches), it doesn't matter how many Earth Spheres you have equipped, 1 or 4; only one will work (just like the Knight's Gloves). I specifically checked this in the Platinum editor. With an Earth, Sorcery, and 40 Magic skill expert, the damage is the same and equals 5692 = (75*40 + 300)*1.5*1.15. If it was different for you, then it's an inexplicable bug.