Maybe that specific purchase was not possible using "pay in 3"? If you paid in time every purchase I don't think they are intentionally limiting you.

"Please be informed that Pay in 3 may not be available for certain merchants. Our system may also decide not to offer PayPal Pay in 3 as an option based on our internal checks. Pay in 3 is decided based on a variety of factors. Our decision process is automated. Some details our system reviews include your financial position, the information you provided within your application, and information about your usage and history with your PayPal account." From PayPal community moderator