It is permissible for you to make up missed Ramadan fasts on the day of doubt and one or two days before Ramadan. For more, please see the detailed answer.

Shab e Barat is a bid`ah and there is no celebration in Sha’ban and there is no special act of worship to be performed in mid-Sha’ban or during the last days of the month. For more, please see the detailed answer.

1- It is recommended to fast three days each month, and it is best to fast al-ayyam al-beed, which are the 13th, 14th and 15th of the month. 2- There is nothing wrong with you fasting three days in Sha’ban, even if some of those happen to be in the latter half of the month.