Maybe you have sorted this out already. With 3 you can buy an internet add on,€4.99 for a month, limit 500MB.
So you could top up by €5 and use it to buy the add on. Or if you top up by €20, you get free limitless internet for a month without using your credit. Once the month is up you can buy the add on, and keep doing every month til your credit runs out. So you would end up getting 5 months for €20.
I don't know all the terms and conditions but most networks need you to make an occasional (maybe every 6 months?) chargeable call to keep the phone number / SIM card active.