Stock mutual funds rose for the third straight week, this time led by mid- and small-cap growth funds. Midcap growth funds climbed 0.58%, hoisting their year-to-date gain to 4.49%, according to Lipper Inc. Small-cap growth funds added 0.56% in the latest week, ended Aug. 28. Their year-to-date return, at -1.67%, was the only long-oriented diversified group still in negative territory.