Is it obligatory to fulfill a vow to fast three days, complete a reading of the Qur’an, and donate one hundred dollars, after the one who made the vow got her menstrual period in the month she vowed in, and became pregnant in the month that followed?

This is a conditional vow. If the condition is not met, then nothing is binding. As long as you did not become pregnant in the month you vowed to perform acts of obedience, then nothing is incumbent upon you because its condition was not met. This type of vow is disliked and brings no good.