A 5/5 battlecry: deal 3 might not even see much play. This is absolute garbage
What? Bull. That would be a slightly better fire elemental, and fire elemental is good.
As far as how this card actually is, it seems roughly borderline playable to me.
Compared to Flanking Strike- this is usually a deal 2 damage to a minion and get a 5/5, which does seem better. Of course, it costs 1 more and needs to actually kill a minion to get the devilsaur, but it can also deal 3 to the face to finish someone off, and can kill a dangerous 3 health minion in a pinch.
I'd guess this is somewhat more likely to not see significant play than so, and it's never going to be an all star, but I'd give it a chance.