[Vireo Dallas]

I will be traveling for 6 weeks traveling in Spain, France and Italy and want to purchase a SIM while still in US. 3 of those 6 weeks will be on Camino de Santiago.

Thank you, love my keyone.

[Bla1ze]

You need to explore the options available in those areas. There's no one SIM that is going to be able to be bought unless you plan on just using your home carrier on roaming. So it's not one SIM you're looking to buy, you're looking to buy one for Spain, France, and Italy.

Spain - Movistar, Orange, Vodafone, Yoigo. Will all work, just depends on pricing for you.
France - Bouygues Télécom, Free, Orange, SFR. Will all work, just depends on pricing for you.
Italy - TIM, Vodafone, Wind, 3. Will all work, just depends on pricing for you.

Most of those places offer pay-as-you go data plans or prepaid SIM's. Most can be bought at airports or in tourist areas. Make sure whatever service you choose, it's loaded up for your time in Camino de Santiago, especially if you need to make reservations or look things up.