Follow the guide here: http://support.giffgaff.com/app/answers/detail/a_id/599/session/L3NpZC9XSDZyYnVoaw%3D%3D
Can't transfer the 3 balance over though I am afraid.
Hi I'm on Three, I would like to transfer my number to Giff Gaff, How Can I Do that? Can I transfer my remaining balance on three to Giff Gaff. I got £ 10.00 on Three.
You can't transfer your balance from Three, you should use your balance up then transfer your number over otherwise you will lose your £10.
To transfer your number you need to get a Pac Code from Three and you have 30 days to put it onto giffgaff or it will expire. When you go to your My giffgaff you will find a link at the bottom of the page to transfer your number to giffgaff. You pick the day you want your transfer to go ahead but sometimes it happens straight away and other times it can take a few hours so try to pick a day that you won't need your phone too much in case it does take a while. It can take up to 9pm on the given day and I should mention that it won't transfer over the weekend so it has to be a weekday.
When u transfer your number you will not get the £10 over. So first you need to get your PAC code, how to do that, go to this website and it will tell you all that you need- http://cow.neondragon.net/index.php/pac-codes-how-to-transfer-your-mobile-number-when-switching-netw...
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