I'm pretty sure this is just a card used by the dungeon run boss, and not an actual collectible one, as if I remember correctly all the uncollectible minions in the dungeon run have gems despite that. (With the exception of the Treasure minions.) I realise that I could be wrong, and therefore am going back and re-watching it to check. If I am wrong, than this is certainly a misconception caused by the fact that when first seeing the card it wouldn't have crossed my mind that it could be collectible based on how useless it'd be in constructed and how overpowered it's be in arena.
Actually, as far as I can see there aren't any "other" non-treasure uncollectibles in the runs, and I can't remember any, (With the excepting of the little rats from the starter boss, which are tokens, so are an invalid tell.) so that heavily suggests that it is a collectible card. (Though Brode didn't comment as such.)
So, uh, this comment contributed nothing, and also goodbye.
Vanilla 2/1 is less than 1 mana worth. Arcane Missiles is worth less than 1 mana. This card is no way worth 3 mana, even if you use only one card instead of two. Would be an incredible card for two mana, excellent for arena, not good enough for constructed. With 3 mana, it is still dust it. As you don't need it in your collection to draft it in arena.