Thank you for your post!
I am sorry to hear Pay in 3 is currently not showing as a payment option for you. I know this must be frustrating. We’re offering PayPal Pay in 3 to a growing number of our UK customers.
PayPal Pay in 3 is not available for certain merchants and goods. We may also decide not to offer PayPal Pay in 3 as a payment option in certain circumstances, such as where our checks suggest an increased risk of fraud or where data we hold suggests you may not be eligible.
This cannot be overridden I'm afraid. All I can suggest is to keep an eye on your payment options when making future transactions. If the option is avilable for you, it will show at the merchants checkout. For further details, please click here.
I hope this helps!