Yup, for 3 days I'd see what your current carrier can offer. If you go into a TIM store, it's going to be around 20 euro for an activated SIM with a little bit of value on it. I think they do have some pay by the day plans, but for 3 days it might be cheaper to go with your current carrier. And on top of that, you have to waste your time finding a store and getting service.