You must stand in place until Kedusha dn then take three steps forward and say kedusha. After that you may sit down.
Explanation:
The Gemara Yoma 53b: R’ Alexandri said in the name of R’ Yehoshua ben Levi one who finishes Shemneh Esrei steps back three steps and then say (Oseh) Shalom. Rav Mordechai said to him, “since he stepped back three steps he must stand there and wait. This is like a student who leaves his Rebbi and walks away immediately, like a dog who eats his regurgitation. A Beratia also says one who finishes Shemoneh Esrei steps back three steps and says “Shalom” and if he doesn’t it was better, he not pray in the first place.(Rashi says it is better he not has been born at all and pray.)
It seems from the Gemara that one is obligated to stand and wait in place after he steps back. So rules the Rif (Brachot 24b)the Rosh, and the Rambam (Tefilla 9:2).
However, it’s not clear how long you must wait after you take three steps back.
The Rif writes you must stand there until the Shliach Tzibbur starts then you may step forward and some say until he reaches Kedusha. The Rambam writes: When the Shliach Tzibbur gets to Kedusha it is permitted for each one to go back to his original position.
Why can’t we walk forward right away?
R’ Chaim Soloveitchik on the Rambam (Tefilla 4:1) writes that one who finishes tefilla and goes straight to his work without asking permission shows that even during tefilla he wasn’t really standing in front of the King of Kings. This Tefilla is not worth anything and it was just a chance encounter where he mumbles through the words. In that case it would have been better he not pray at all.
The Shulchan Aruch rules like the Rambam and the Rif (OH 123:2) writes: he should stand in the place where he finished his three steps and should not walk forward until the Shliach Tzibbur gets to Kedusha, or at least until he starts Chazarat Hashatz.
The Divrei Chamudot writes that when we say long Piyutim before Kedusha then we don’t have to wait only until the Chazan starts Chazarat Hashatz then one may go back forward. So says the Mishna Berura. So too any other great need one may only wait until he begins.
All the Poskim delve into how long one must stay where he is after Shemoneh Esrei, however there is not much discussion of one who walks away from his place, does he need to go back for Chazarat Hashatz. The Kesef Mishne writes that he is not required to go back to his place rather he may answer Kedusha from where he is. So quotes the Mishna Berura.
The Kaf HaChaim writes it is a wonder that people are so lenient to sit after Shemneh Esrei and to stand when the Chazzan starts. It is against a straight halacha they must stand where they are at least until the Shliach Tzibbur. It Shulchan Aruch good idea to remind the nation foot of this halacha.
One other issue I would like to bring up is how we step back three steps. The Shulchan Aruch (123:1) writes: One bows and walks back three steps with one bow and afterwards while he is bowing should say Oseh Shalom and turn towards left, Hu yaaseh… and turn left and then bow low like a servant bowing before his master.
The Shulchan Aruch writes that when we step backwards, we should bow and then step back. When we turn right and left, we should not straighten up until after we bow forwards. May people walk backwards straight and just turn their faces left and right and this is wrong.
