you can do a balance transfer from the egg card to the new one, but you will have to pay a fee to do that. Check what interest rates they charge for balance transfers too.
The 3% is easy to work out - divide your balance by 100 and times by three to get the minimum payment. They will send you a bill and tell you how much to pay anyway
0% for 3 months means just that - if , for example you got your card 1st august, you can buy stuff, and just be charged for the stuff, no interest till november 1st. On nov 1st you will start paying interest. If you bought something nov 2nd and after, interest would be charged immediately (well, you next statment)