Hmm, I'd go with rats. Not the small cute ones you hold in a cage, but the huge rats which thrife in sewers. Imagine their social structure would be kept intact despite having human intelligence...they'd start hunt us down as food and populate themselves reaaaaally fast
Hm....imagine a bear 3 times bigger. That could get scary.
How could I forget this huge piece of "what the hell???"
Probably ants, they'd take over the world easily. Maybe rats too.
How has nobody mentioned a Gorilla yet?
My vote is on ants, there are just so many of them, if you made them bigger and stronger and increased their intelligence, it would look rather bleak for us.
I would be scared of centipedes. I'm already scared of them. Them being 3x the size would be horrifying.