I'm not a big fan of the "by our powers combined" set, but I'll grudgingly give credit where credit is due, this is a good but expensive card.
3 Cost is steep, this is the major issue for this bad boy, but here's what you get:
First up, it's a fight action, so 1 damage base, that then means that with just 2 classes in play it's a 1xp, 3 cost attack at +2 for 3 damage. If you could take a 1XP Backstab with a +2 on it, people would most definitely do that. Already ignoring the whole setup thing of gathering all the classes, even with just 2 classes in play, the usual minimum a character can muster, this card is playable.
Everything on top of that is gravy. Do you manage to plop down a third class via a Dual class card? A fourth class through judicious dual class design or a triple class card? Are people trading cards with Black Market, Bob Jenkins, Teamwork or similar effects? The third class is relatively easy, Enchanted Blade by itself will get you a third class for most characters. Remember that your character is a card and it's included, so if a dude like "Skids" O'Toole plops down Enchanted Blade that means you've got 3 classes in play for an effective +3 and 4 damage and that right there by itself is gold. 4 Damage attacks are rare and powerful and worth the hoops to jump through for.
Can you max it consistently? That'll be rare. But a partial trigger for just +3 or +4 damage is excellent.