The Internal Revenue Service said today that the total number of tax returns received by the agency for the tax season through March 12 fell by 2.9 percent as the number of returns electronically filed rose by .2 percent. Meanwhile, taxpayers were turning away from professional preparers and towards self-prepared returns. All these trends have been viewed as a reaction to economic conditions.While the IRS termed the drop slight, it was greater than had been predicted by many observers before the season began in January.