It looks like you are using only Gaussian function to approximate the peak. In reality you see mixtures of Gaussian function and Lorentzian function, which are combined into Voigt function. That is, you do not have three peaks, only one peak, but a different lineshape. More details in https://pubs.aip.org/avs/jva/article/38/6/061203/1023652/Practical-guide-for-curve-fitting-in-x-ray#88895297.

Mikhail Zimin Thank you so much for your answer.