I'm 5'7" w/o heels, and I ride a 50cm BareKnuckle. They've been said to run large as well (whatever that means) but the frame itself is just about perfect as far as sizing goes.

Pass on the 3ren and get a frame that fits, if you're going to do anything even coming close to serious cycling, you'll be happier for it, no matter what deal you might be getting on the 3ren.