The term Transmute refers to an action performed by Kanai’s Cube. For example: Extracting a Legendary Power from an item, reforging a Legendary item, upgrading a Level 70 Rare item to a Legendary item, etc.
If I understand you correctly, your “transmutation” has already been performed. And what you are actually trying to do is “equip” Legendary Powers in your Cube Slots.
Season 20’s Theme allows you to equip any 3 Legendary Powers without restrictions. That is to say: You could, if you wanted to, equip 3 Weapon Powers, 3 Armor Powers, 3 Jewelry Powers or any combination of these.
So, your Seasonal Necromancer on the Americas Region should be able to equip In-geom in the Weapon Slot, Nemesis Bracers in the Armor Slot and Leoric’s Crown in the Jewelry Slot, provided you have extracted the Legendary Powers from each of those items.
Note: The Legendary Powers are listed in the Cube in alphabetical order of the Legendary item’s name. (Aquila Cuirass will be listed before Leoric’s Crown).