@Add_198031 Hi there & welcome to the 3Community. Can I get you to try the below settings & let me know if you're able to add 3Billing back onto your Play Store account?
When you're in the Payment methods page where you can add the 3Billing can you select 'More payment settings'? You'll be redirected to the Google Pay website, in the top-right corner you'll see your account icon, can you click on the icon & then if you have more than one email address make sure you have the correct one selected. You may only have one email address listed & if you do all you need to select is 'Manage your Google Account'.
You'll see at the top of the screen a side scroll bar with Home & Personal info, scroll the bar over until you see Payments & subscriptions, in this section, you'll see payment methods & manage Payment Methods. You should see 3Pay listed with an option to remove it. Can you remove the payment option & once that's done go back to the Play store & try to add the 3Pay again. Let me know how you get on?
