Andrea,
There are two basic options you could consider:
1.) Purchase inexpensive PAYG phones when you arrive in France. There are lots of mobile phone shops in Paris, and you'll probably find some at the airport. Be sure that you're clear on the method for "topping up".
2.) Buy or rent a phone from one of the travel phone firms, such as Roam Simple, Call In Europe, Cellular Abroad, Mobal or Telestial.
As you're only going to be in France for 12-days, buying three phones off E-Bay and just buying SIM cards in France might not be the best choice, unless you're planning to use them on future trips.
Happy travels!