Yep, that's what I did too. I just kept changing the blankets on their bed constantly. I bought up all the baby blankets at one weekend of garage sales that I could find.
Then they just gradually caught on. They would not potty by their food bowls and not by their bed. Everything else was covered with pee pads. Everyone that took a puppy at 12-15 weeks reported back that they were already house-trained. I felt like they were here, but was not sure if they would make the transition, but they did with few accidents. I kept the last one, Rocky and he is doing great! The only one I have to worry about is Ben -- he still is marking occasionally. It is getting better but he may lose a couple of things soon if he does not stop.