Turning one’s face around when the congregation is praying Shemoneh Esrei:
An individual who prayed together with the congregation and completed his Shemoneh Esrei prior to the congregation, does not have permission to turn his face around, to face the congregation [who is standing behind him], until the Chazan completes his prayer, in which case he may turn his face around. However, his feet must remain in place until the Chazan reaches Kedusha, as explained elsewhere. [Various reasons are mentioned behind why one may not turn around to face the congregation, and each reason carries its own Halachic ramifications. In accounting with all the reason, one should not face the congregation, even if he did not Daven with the congregation. Likewise, some say one should not turn around until majority of the congregation completes the Shemoneh Esrei. However, from the Rama and Admur ibid it is implied that this law only applies to one who prayed with the congregation, and only until the Chazan finishes his prayers. Vetzaruch Iyun.]
During a Minyan, it is forbidden for one to turn around after Shemoneh Esrei until the Chazan finishes his Shemoneh Esrei. Some are stringent to not turn around until majority of the congregation finishes.
May one turn around to see if he can take three steps back?
Yes. One may take a swift glance behidn him to make sure that he can take trhee steps back at the end of his Shemoneh esrei, even if the Chazan has yet to complete his prayer.
 Admur 123:5; Rama 123:2 “It is forbidden to turn his face around, to face the congregation until the Chazan finishes his prayer”; Beis Yosef 123 in name of Shibuleiy Haleket 25; M”A 123:9; Taz 123:5; Soles Belula 123:2; Shalmei Tzibur p. 136; Beis Oveid 10; Chesed Lealafim 123:24; Kaf Hachaim [Falagi] 15:46; Chayeh Adam 24:30; Derech Hachaim 5; Kitzur SHU”A 18:13; M”B 123:12; Kaf Hachaim 123:22
Other opinions: Some Poskim question that seemingly it should be forbidden to turn one’s face around until the Chazan reaches Kedusha, just as is the law regarding returning to one’s place. [Bach 123, brought and negated by Taz 123:5 and M”A ibid]
 Taz 123:5
 Admur ibid; Rama ibid; P”M 123 A”A 9
Other opinions: Some Poskim write that only when the Chazan begins the Chazaras Hashatz may one turn around. [M”A ibid; See P”M ibid] Other Poskim rule one may not turn around so long as majority of the congregation is still Davening. [P”M ibid and Biur Halacha 123:2 “Assur”, in accordance to 2nd reason in Levush 123:7, brought in Taz 123:5, but omitted from M”A ibid; See wording in Shibuleiy Haleket ibid] The Biur Halacha ibid concludes with a Tzaruch Iyun
 Admur ibid; M”A ibid; Taz ibid; Kneses Hagedola 123; Olas Tamid 123:2
 The reason: It is forbidden to turn around prior to the completion of the Chazan, in order so people do not suspect that he skipped parts of the Davening. [1st reason in Levush 123:7, brought in M”A 123:9; See Mamar Mordechai 123:5; Machatzis Hashekel ibid; Levushei Serud 123:6; 2nd reason in M”B ibid] Alternatively, the reason is because turning around will cause those who are behind him to lose their concentration, upon looking at them. [2nd reason in Levush ibid; Taz 123:5; 1st reason in M”B ibid] Alternatively, one may not turn around as the people in front of him who are still praying, have the Shechina reside by them, and it is improper to turn one’s face away from the Shechina. [3rd reason in Levush ibid, brought in P”M 123 M”Z 5] According to the last two reasons, the matter is seemingly not dependent only on the conclusion of the Chazan, but also on the congregation, and hence one may not turn around until majority of the congregation has completed their prayers. However, according to the first reason, it is allowed. [P”M 123 M”Z ibid and A”A ibid; Biur Halacha 123:2 “Assur”]
 The reason: As doing so will cause them to lose concentration, and one is turning his face from the Shechina.
 P”M ibid and Biur Halacha 123:2 “Assur”, in accordance to 2nd reason in Levush 123:7, brought in Taz 123:5, but omitted from M”A ibid; The Biur Halacha ibid concludes with a Tzaruch Iyun
 Maaseh Rav 21; Orach Neman 3 p. 142; Daas Notah of Rva Chaim Kanievsky p. 336