I have an extra iPhone 3Gs which I am not using since I have a iPhone 5. I would like to factory unlock it because I will be in Italy for two weeks and would like to buy a sim card to put in my iPhone 3s to use while I am there. Can I do that?

I will be going to Italy next month and would like to have access to my cell phone without incurring roaming charges, therefore, I would like to buy a sim card to use in an extra iphone I have that I am not using since I have the iphone 5 under AT&T as my carrier. The iphone 3Gs has no phone service but works with Wifi. Will I be able to factory unlock the iphone 3s so that I can purchase an Italian sim card to make phone calls while I am there? Is that possible if it does not have a carrier service?