Champion Shards

So I've been saving champion shards in hopes of getting three shards (which should be enough to buy the champion if I understand right) But when I tried using the three champion shards to upgrade to a Champion permanent from my Loot tab, there's no option to upgrade it.

I figure I could just disenchant them all to grab the champion, but wouldn't it make sense to have that option in the Loot tab? Or am I missing the option somehow? (Aside from just disenchanting and purchasing it from the shop)