In the wake of winter weather that shut down University operations on three separate days this semester, some Harvard Square businesses have experienced a downturn in sales and customer traffic this season. Business owners pointed to recent MBTA service suspensions as well as parking and travel bans as factors inhibiting employees and customers from travelling to the Square. “Businesses have reported that it has been very difficult these last few weeks with the snow,” said Denise A. Jillson, executive director of the Harvard Square Business Association.