Siman 293
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מְאַחֲרִין תְּפִלַּת עַרְבִית, כְּדֵי לְהוֹסִיף מֵחֹל עַל הַקֹּדֶשׁ.
1 The Laws of the Evening Service on Motzaei Shabbat, 3 Seifim: We draw out the Maariv prayer in order to add holiness to the regular week.
ב צָרִיךְ לִזָּהֵר מִלַּעֲשׂוֹת מְלָאכָה עַד שֶׁיֵּרָאוּ שְׁלשָׁה כּוֹכָבִים קְטַנִּים, וְלֹא יִהְיוּ מְפֻזָּרִים אֶלָּא רְצוּפִים, וְאִם הוּא יוֹם הַמְעֻנָּן יַמְתִּין עַד שֶׁיֵּצֵא הַסָפֵק מִלִּבּוֹ.
2 One must be careful not to do work until he sees three small stars that are not scattered, but rather in a row in one place [in the sky]. If it is a cloudy day, one should wait until he is no longer uncertain at all [that three stars will have appeared].
ג מִי שֶׁהוּא אָנוּס, כְּגוֹן שֶׁצָּרִיךְ לְהַחְשִׁיךְ עַל הַתְּחוּם לִדְבַר מִצְוָה, יָכוֹל לְהִתְפַּלֵּל שֶׁל מוֹצָאֵי שַׁבָּת מִפְּלַג הַמִּנְחָה וּלְמַעְלָה וּלְהַבְדִּיל מִיָּד, אֲבָל לֹא יְבָרֵךְ עַל הַנֵּר, וְכֵן אָסוּר בַּעֲשִׂיַּת מְלָאכָה, עַד צֵאת הַכּוֹכָבִים. הַגָּה: וְנוֹהֲגִים לוֹמַר: וְהוּא רַחוּם וּבָרְכוּ בַּאֲרִיכוּת נֹעַם, כְּדֵי לְהוֹסִיף מֵחֹל עַל הַקֹּדֶשׁ (אוֹר זָרוּעַ).
3 A person who is prevented (from reciting Havdalah), such as one needs to go beyond the techum to perform a mitzvah can pray the prayers for Motzaei Shabbos from the time of Plag HaMinchah and may make Havdalah immediately. However, in this case one does not make the blessing over the candle; and it is still forbidden to perform work until the stars come out. Rema: It is our custom to say והוא רחום and ברכו with protracted, melodious tones so as to add from the mundane to the holy. (Or Zarua)