No, you should park at Epcot parking lot if you want to go to Epcot for the day.
Parking at the Boardwalk is for guests of the resort or for those visiting the Boardwalk, not Epcot or MGM, for shopping or eating.
Most resorts that are near a park have a 3 hour limit as well and will give you a parking permit. At least that's the case at the monorail resorts for the same reasons.