Q: how to rent movie on Apple TV (3 Gen) using credit balance in iTunes account

Hello Geneber,

Thanks for using Apple Support Communities.

If you have already redeemed your iTunes Store gift card, then the balance should be able to be viewed on your iTunes Store account.  As long as you are signed in with the same Apple ID on your Apple TV, the purchase on the Apple TV will be deducted from your credit balance.

iTunes Gift balances - Apple Support

Take care,

Alex H.