Must I make up the three days I fasted based on an incorrect imsakiyya that was one hour too early?

Whoever breaks their fast during the day in Ramadan, thinking the sun has set, and then realizes their assumption was wrong, must make up that day, according to the majority of scholars, including the four imams. They cite as evidence what Al-Bukhari narrated from Asma bint Abi Bakr (may Allah be pleased with her), who said: "We broke our fast during the time of the Prophet (peace be upon him) on a cloudy day, then the sun appeared." Hisham (the narrator of the hadith) was asked, "Were they commanded to make up the day?" He replied, "There must be a making up."

Likewise, Malik narrated in Al-Muwatta that Umar ibn Al-Khattab broke his fast one day in Ramadan on a cloudy day, thinking it was evening and the sun had set. Then a man came to him and said, "O Commander of the Faithful, the sun has appeared." Umar said, "The matter is easy, and we have exerted our effort." Malik interpreted his saying, "The matter is easy," to mean making up the day.

Therefore, if you broke your fast before sunset based on an incorrect timetable, you must make up those days. As for abstaining from food and drink an hour or more before Fajr, this does not necessitate making up the fast, and it is better to delay it until the last part of the night.