You can check your Pay As You Go credit and Add-on allowances any time you like by calling 444 free from your Three phone or by going online to My3, either on your phone or computer.
To check your balance using:
My3 on your Three phone
My3 on your computer
Or call 444 on your Three phone or 0843 373 4444 (national rate). Calling 444 is free on your Three phone so you can use this option even when you have no credit left.