Originally, the game was available as a digital purchase on the PlayStation Store for the PS3. However, as Sony has shifted focus to the PlayStation 4 and 5, and with the eventual closure of the PS3, PSP, and Vita stores (though the PS3 store remains operational for now, purchasing new digital titles has become more complex), finding a direct digital download code can be difficult.
After purchase, go to your Game Library > Purchased. Download Uncharted 3- Drake-s Deception