INGLEWOOD, Calif., Feb. 15— Earvin (Magic) Johnson scored 39 points and led Los Angeles back from a 17-point third-quarter deficit to a 106-103 victory over the Boston Celtics, giving the Lakers the best record in the National Basketball Association. Johnson scored 8 points over the final three and a half minutes, including two clinching free throws with four seconds to play, to lift the Lakers to their 38th victory in 50 games this season, one game better than the Celtics. After the teams exchanged baskets, Johnson sank an 18-footer with 23 seconds to play to put the Lakers ahead, 102-99. After Robert Parish scored on a dunk, Kareem Abdul-Jabbar made two free throws to put the Lakers up by 104-101 with 16 seconds to play. The free throws gave Abdul-Jabbar 10 points, keeping alive his streak of 743 consecutive games in double figures.