Three-Cornered Hats is a classical method to obtain the frequency stability of one single atomic clock without more stable frequency standards available. In this paper, the measurement data of three hydrogen masers in a ground station is used to analyze the frequency stability of one single atomic clock. The analysis steps and problems in application of this method are introduced. The negative values of the calculated Allan variance of one clock with averaging time larger than 4000s occur. It probably implies that the hypothesis of uncorrelated clocks is not acceptable, which will be analyzed further.