This is presently only available to customers in certain territories via "PayPal Pay Later" (maybe referred to as a different name in your country).
Not all customers will be eligible for it and it is only eligible between purchases of £30 and £2000.
Terms and Conditions apply, please see PayPal's website here for more details.
Please note that you will need to utilise a personal PayPal account, as Pay in 3 is not an available option on PayPal Business accounts.