BOSTON — March tax collections in Massachusetts fell slightly compared to the same month last year, but were still ahead of expectations for the month by about 8.5 percent.Revenue Commissioner Christopher Harding said Wednesday that the state collected $2.24 billion last month, which was $176 million above predictions for March.Harding said state revenue officials had anticipated that corporate and business taxpayers would take advantage of a recent change in the corporate filing schedule that shifted the March reporting deadline to April.