The decision process is automated. PayPal assess your financial position using a combination of the information you provided in your application and information about your usage and history with PayPal to decide if PayPal Pay in 3 is suitable for you now.
They may also run a "soft" credit check when you apply if PayPal need more information to be able to make a decision. This search will only be visible to you and will not affect your credit score.
If you apply during check out and are declined for PayPal Pay in 3, you can still use PayPal to check out securely with your debit or credit card.
If you want to re-apply, you will be able to do so 35 days after your last application.