Hello.
Welcome to the Three Community.
If I've understood correctly, you're looking to have the allowance from the auto renew data pack each month, rather than having credit on the account. If so, you don't need to auto top up each month.
When you select an auto renew data pack, the system will automatically purchase the chosen data pack each month when the old one runs out. The system will ask you when you first choose the auto renew, how would you like to pay for the pack, you can use credit, or a registered payment card. In this case, you'd choose your card. This will continue until you stop it.
If for some reason you did have auto top up active as well, it's likely you'd end up with credit on your account plus the selected auto renew, this is because the system would not change the chosen payment method from card to credit.
Hope this makes sense.
Pete.
