Is it a condition to fast three days of every month, starting from Muharram until Dhul-Hijjah, to obtain the reward of fasting a year, and is the reward lost if one does not fast in one of the months?

Fasting three days every month is equivalent to fasting a lifetime, because a good deed is rewarded tenfold. It is not necessary for the fasting to begin in Muharram. Whenever you fast three days out of every month, you will attain this virtue. Some scholars have mentioned that if one misses fasting three days every month, they can make them up at another time to complete the total, just as the Prophet, peace and blessings be upon him, used to make up his fasting in Sha'ban if something prevented him from it.