What is the ruling on someone who vowed to read three hours daily, then fell short three times: is he obligated to complete the missed hours, or to give in charity the amount vowed?

If by reading, the recitation of the Quran is meant, then its recitation is among the acts of drawing closer to Allah. Your vow to read for three hours every day is binding and must be fulfilled, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him."