PSG host relegation battling Le Havre on Saturday
The champions, who started the game level with Sevilla and Atletico Madrid on 28 points, swept to victory in Cornella as Messi put on a sublime show in the week he was ranked the fifth best player in the world at Monday's Ballon d'Or awards.
Luis Suarez added the third just before halftime with a shot from a tight angle that deflected in off goalkeeper Diego Lopez.
Espanyol's fourth consecutive league defeat left them ninth in the standings.