A powertrain of an automatic transmission includes a first planetary gear set having first, second, and third operating members, a second planetary gear set having fourth, fifth, and sixth operating members, and third planetary gear set having seventh, eighth, and ninth operating members, the fourth and seventh operating members being fixedly interconnected, the fifth and ninth operating members being fixedly interconnected, the third and eighth operating members being selectively interconnected, the second and seventh operating members being selectively interconnected, the second and sixth operating members being selectively interconnected, wherein a transmission case is fixedly connected to the first operating member and selectively connected to the seventh and eighth operating members, an input shaft is fixedly connected to the third operating member and selectively connected to the seventh operating member, and an output shaft is fixedly connected to the ninth operating member.