It appears the Year 3 version of the Turris Shade mark from Spire of Stars is not able to equip the Prestige ornament. I have the ornament unlocked in collections and even reacquired it and used it again from my mod menu but it still won't appear in the ornament list, only showing universal ornaments. I'm not sure if any other Leviathan Lair armor lacks the ability to use their ornaments in Year 3, but the ornamented version of the mark was a part of my favorite fashion from last year so I'm particularly invested in getting to use this skin of it in the future.