3 bills at the end of your billing cycle whilst Vodafone bills in advance, at the start of your billing cycle. This means when you move across from 3 to Vodafone you will receive your final bill from 3, then your first bill from Vodafone. If you check the dates these bills are for, you should find your Vodafone bill takes over from the date your 3 bill finished. There is a section on our website that outlines this.
If you think that you've been billed incorrectly, or that your bills overlap, please let me know and I can look in to it for you.