I'm getting an error message apple pay. An error occurred while setting up apple pay" but I've been using it for a year I have 3 credit cards in the wallet. It's new problem. I can't add additional cards using iOS 9.3.4

I will paraphrase what I think you're saying, because your post is not entirely clear to me.

• You have been using Apple Pay for a long time, with three cards in your wallet.
• You now have iOS 9.3.4.
• You have not tried to use Apple Pay since installing iOS 9.3.4, so you don't know if it works with this iOS release.
• You are now trying to add a fourth credit card to your wallet, (at Settings > Wallet & Apple Pay > Add Credit or Debit Card) and are receiving an error message "could not set up Apple Pay ..." while attempting to add the card.

If that is the scenario, is your storage full? Check Settings > General > Storage & iCloud Usage > Manage Storage (under Storage) and be certain you have absolutely no less than 10% of your iPhone's total storage free. I would also assume you are in the country you reside in, and that has not changed. I assume you are logged in to your Apple ID correctly at Settings > iTunes & App Stores as well as Settings > iCloud.

Make sure additional cards are supported bank cards.