With which foot is one to take three steps back after Shemoneh Esrei?
When one takes three steps back after Shemoneh Esrei, he is to begin with his left foot.
 Admur 123:5; Michaber 123:3; Hagahos Maimanis Tefila 5 Samech; Shibulei Haleket 18 in name of Midrash Socher Tov 35b; Levush 123; Perisha 123:1; Ateres Zekeinim 123:2; Birkeiy Yosef 123:5; Ketzos Hashulchan 20:24
Other opinions: Some Poskim rule one is to first lift his right foot. [Maharil in name of Orchos Chaim, brought in Beis Yosef 123, based on Yuma 55b that all turns should be done to the right]
 The reason: As a person naturally lifts his right foot first upon walking, therefore, in this case he should first lift his left foot to show that it is difficult for him to part from Hashem. [Admur ibid; 1st reason in M”A 123:10] Alternatively, the reason for this is because the Shechina is found by the right side, and we hence give honor to the right. [2nd reason in M”A ibid; Taz 123:6; Bach 123; Peri Chadash 123:2] The practical ramifications between the two reasons is regarding a lefty. See M”A ibid; Kaf Hachaium 123:23
 Admur ibid; 1st approach in M”A 123:10; Ketzos Hashulchan 20:24; Implication of M”B 123:13 and Biur Halacha 123 “Kesheposeia”
Other opinions: Some Poskim rule that even a left footed individual is to first lift his left foot [2nd approach in M”A 123:10; Elya Raba 123:8; P”M 123 A”A 10; M”Z 6; Derech Hachaim 3; Aruch Hashulchan 123:7; Orech Neman 123:10; Kaf Hachaim 123:23]
 This can be determined based on what foot he begins walking with [see Rashi Bechoros 45b; Noda Beyehuda Tinyana E.H. 155; Ginas Veradim 1:9] or which foot he uses to kick a ball, or to go up a flight of stairs. [See Zechor Leavraham 1 Chalitza 12]
 The reason: As since a left footed individual naturally lifts his left foot first upon walking, therefore, in this case he should first lift his right foot to show that it is difficult for him to part from Hashem. [Admur ibid]