Prepaid offers are called "Ricaricabili" on the 3 website. As the date of writing, there's 3 offers - Super 7, Super 10 and Super 0 Mondo.

Important Note: In November 2008 I tried without success to buy a prepaid SIM card from 3 in two stores in Rome. In both stores they were asking for a codice fiscale (Italian tax number) without which they said they could not sell a prepaid SIM. I have this tax number but I didn't bring it on an official document. If you are determined, bring an Italian friend who has the necessary documents. Otherwise, pick up a SIM from WIND or TIM, who also have interesting prepaid offers for Internet access and do not want to see an Italian tax number.

Settings
Use this page to send 3 settings to your phone.

Manual Configuration
APN: tre.it
Username: guest
Password: guest

WWW3
This option allows mobile web browsing using 3 network. It costs 1€ daily, 2€ weekly or 6€ monthly and has an allowance of 50MB daily.
You should use 3 mobile browser which can be sent to your phone from this page.

Naviga 3
This option allows PC web browsing using 3 network. It costs 3€ weekly and has an allowance of 100MB daily or 9€ monthly and has an allowance of 50MB daily. Extra MB off bundle cost 0,60€. This allows full access to internet. The Software FastMobile Modem3 is not nessasary (it´s a XP/Vista-Software to connect your Mobile Phone to the PC/Laptop).
To activate:
1. Call 4311 and select option 2; (actual Webpage May/2008 says Call 4321 and select option 2)
2. Access USIM management section Area Clienti 133.

Self Service via Web-Access
You can access the "Area Clienti" self service via Web-Browser.
a) goto http://areaclienti.tre.it/selfcare/areaclienti133/area_index_ITA_HTML.xsl
b) Type in your Mobile Number (Prefisso: 392 - Numero: the number after 392)
c) Click on "Conferma"
d) Wait until you receive your temporarily password as SMS
e) goto http://areaclienti.tre.it/selfcare/areaclienti133/4552_infoCosti_ITA_HTML.xsl(Info costi)
f) enter your temporarily at "nserisci password"
g) enter your new passwort at "Nuova password" and confirm it at "Conferma password"
Your passwort must have letters and numbers!
h) You can enter a password-receovery Questtion (domanda) and Answer (risposta) if you loose your password.
i)Click on "Conferma"

Now you can access your Credit-Information via this Link:
http://areaclienti.tre.it/selfcare/areaclienti133/4552_infoCosti_ITA_LOGGED.xsl
For detailed billing information you can select in the "Visualizza dettaglio traffico"-Area
for last day "giorno precedente" or for last week "ultima settimana" and then click "invia"

If you want to activate any options you must goto Gestione Usim:
http://areaclienti.tre.it/selfcare/areaclienti133/4574_usim_ITA_LOGGED.xsl
Take a special look to "Naviga 3 30 gg". Here you can deactivate/activate the automatic renewal of this option for the next month ("Rinnova automaticamente")

Recharge via Internet
You can recharge / top-up your card with CreditCard or PayPal.
Go to this page: https://www.shoptre.it/shop3/charge_process.php
Goto "2.) Inserisci tuoi dati" - input your phone number and select (scegli) CreditCard or paypal