Hello Jinbo_Huang
Yes, because it is 3 days and the Swiss travel pass is valid from the 1st day of travel and for the 2 following days. The Swiss travel pass flex is a subscription where you can use the number of days within 30 days. However, this is also not bound to, for example, the month of December, but from the date of validity.
Have a nice day.
Kind regards, VlaznimA.