I just use the repeating resource section, add one called "Attunement" and add 1 to it each time you attune an item. Not sure if there's an automated API way to track an item with the string "requires attunement" in inventory? Even then, there's nothing stopping you from carrying more than 3, so it wouldn't be that reliable anyway. You could also just edit the item name in the inventory and chuck a ⭕ or similar at the start of the description when the item is attuned during a rest. Like a prepared spell. Hmmm... actually I like this, I might start using it!