Is Asada Mao the only figure skater that can do the triple axle?

No. The triple axle is fairly common. I believe you are thinking of the quad. axle, (four turns).

Actually, the triple axle is not common in woman's figure skating.

The triple axle is 3.5 turns, and is the most difficult single jump for woman's.

Quads are challanged by men's, though female skaters such as Ando Miki have challanged it in the past.