If you are on 3 Prepay 20 you have 19GB of data which can be used for roaming in the EU, even though the UK has left the EU this allowance will still work there. The image above is out of date click here to have a look at the latest version.
Before you travel I would recommend having a read through our blog All About Roaming with Three, it has some handy tips and tricks for when roaming.