In sefer minhat yehudah (and brought down in many other works) he writes one can be metakken certain sins by fasting 4 times a year for three days straight, back to back; once before rosh hashana , before kippur, before 10 tevet , 17 tamuz
q.1 does this mean the days immeadiatley before?
q.2 if shabbat seperates does the fasting thursday-fri-sunday count as ‘back to back’
q.does this have to be day and night or just the fast during the day and then one would be allowed to eat at night?
I would have to see the source inside to be ceratain, but in general, the special segulah of three “back to back” fasts refers to three consecutive days and nights. According to the Arizal, fasting for three consecutive days and nights is equivalent to 40 (consecutive) regular fasts, and this is a well-known segular for atoning for different sins.
Because this is not very easy, some prefer to fast for two consecutive days and nights (equivalent to 27 fasts), and to continue to fast another thirteen consecutive days (just the day, not including Shabbos, Rosh Chodesh, etc.) to complete the 40 fasts.
For three consecutive days and nights, Shabbos cannot interrupt.
It does not refer specifically to the “days before,” and in general the segulah of 40 (or 84, and so on) fasts is good anytime, though there are times when it is customary or especially worthy to fast (such as the shovavim time).