House Training an Abused Dog?

I recently adopted a girl Shih Tzu mix. She was badly abused. I've been trying to take her outside every 3 hours, but I still find poop in my boyfriend's kids bedroom, on their bed, or on my bed. When she poops in the bed I call her a bad dog and I will flick her on the nose. I also put her outside thinking she will learn her lesson, but it's constantly going on.

If she is not pooping in the bedroom or on our beds then the she is peeing in the bathroom on the floor mat. I don't know what I can do, but I keep trying. When I do take her outside she only pees, but when she does poop outside I give her a Milkbone as a reward. Please help me potty train her.

By Bajanbeauty