If all 3 phones are being used, each of you will need to buy a sim card. With TIM, you need to pay €10 per sim which includes €5 of credit.
Two of you could get the most basic plan which I cannot recall the name. Under the basic plan you are only charged something like 10-15 cents per minute for talk and maybe 5 cents per text. If you just use these 2 phones only for emergencies, the €5 maybe enough for your whole trip. You can always add more credit. You can compare this to your roaming charges or a travel plan from your provider and decide which is more economical, which will all depend upon your usage. If you run out of credit, you will not be able to make calls or texts and you will need to go to a TIM to add credit.
One of you can add the Welcome option which is available for tourists (bring your passport). For extra €20, you get 200 included minutes and 2 GB of data. The included minutes may be used for free international calls. And you can tether or create a hotspot to share the data with a nearby phone, tablet or computer. Text is not included, but will be charged at about 15 cents per text using the €5 credit. The Welcome plan is valid for 30 days and only in Italy. If you leave Italy, you will need to pay roaming charges which will come out of your credit. I reccomend Welcome. I used it for a 19 day trip.
You need to go to the TIM website to figure this out and to get the exact pricing for calls and texts. It is in Italian, but there should be an English pdf which gives you the basic info.
If you do decide to do this, find a good English speaking TIM dealer and get them to set up everything for you on your phone before you use it. TIM will send you italian text messages every once in a while and I accidently activated an SMS notification which sent me a text each time I made a call. I tried to call TIM helpline, but they only spoke Italian. The SMS texts started using up my credits until I got the dealer to sort things out for me.