Shulchan Arukh, Orach Chayim 109

<b>דין איך יתנהג היחיד לכוין בתפלתו עם הצבו' ובו ג סעיפים:</b><br>הנכנס לבה"כ ומצא צבור מתפללין אם יכול להתחיל ולגמור קודם שיגיע ש"צ לקדושה או לקדיש יתפלל [וה"ה אמן דהאל הקדוש ושומע תפלה דינן כקדיש וקדושה] [תוס' והרא"ש ומרדכי פ' מי שמתו ותרומת הדשן סי' י"א] ואם לאו אל יתפלל אם אין השעה עוברת ואם נכנס אחר קדושה אם יכול להתחיל ולגמור קודם שיגיע ש"ץ למודים יתפלל ואם לאו אל יתפלל וה"ה אם יכול להגיע למודים או לאחת מהברכות ששוחים בהם כשיגיע ש"צ למודים יתפלל ואם צריך להתחיל כדי לסמוך גאולה לתפלה ונזדמן לו שמגיע ש"צ למודים כשהוא באחת מהברכות (באמצעה) ישחה עמו אבל אם הוא בתחלתה או בסופה לא ישחה שאין שוחין בתחל' ברכה או בסופה אלא באבות ובהודאה:

The Law Regarding How An Individual Should Conduct Themselves In Order To Focus Their Prayer [Together] With The Congregation, Containing 3 Se'ifim: One who enters the synagogue and finds the congregation praying, if one is able to start and finish [one's Amidah] before the the prayer leader arrives at Kedushah [the third blessing of the Amidah] or Kaddish, one should pray. [And this is [also] the ruling for [answering] Amen to [the blessings of] "The Holy One" [at the end of the third Amidah blessing] and "Who Hears Prayer" [at the end of the third-to-last Amidah blessing]; their law is like Kaddish and Kedusha [Tosefot, the Ro"sh, and the Mordecai in the chapter "Mi Shemeito" [Tractate Berakhot, chp. 3], and Terumat Hadeshen siman 11] and if not, one should not pray if [i.e., as long as] the time [for praying the Amidah] has not [yet] passed. And if one entered after [the congregation recited] Kedushah, if one is able to start and finish [one's Amidah] before the prayer leader arrives at Modim [the second-to-last blessing of the Amidah], one should pray; and if not, one should not pray. And this is the ruling if [as long as] one is able to arrive at Modim or one of the other blessings in which one bows: when the prayer leader arrives at Modim, one should pray. And if one needs to begin [praying the Amidah] in order to juxtapose Geulah [the blessing of redemption that immediately precedes the Amidah] to the [Amidah] prayer, and it happens that the prayer leader reaches Modim when one is in the middle of one of the [Amidah] blessings, one should bow with [the prayer leader]. But if one is at the beginning or end [of one of the Amidah blessings], one should not bow, because we do not bow at the beginning or end of a blessing, except for Avot [the first blessing of the Amidah] and Hoda'ah [the second-to-last blessing of the Amidah].

אם מתחיל להתפלל עם ש"ץ כשיגיע עם ש"ץ לנקדישך יאמר עמו מלה במלה כל הקדוש' כמו שהוא אומר וכן יאמר עמו מלה במל' ברכ' האל הקדוש וברכ' שומע תפל' וגם יכוין שכשיגיע ש"ץ למודי' יגיע גם הוא למודי' או להטוב שמך ולך נאה להודו' כדי שישח' עם הש"ץ במודים: הגה אבל לכתחלה לא יתחיל עד אחר שאמר קדושה והאל הקדוש אלא שאם הוצרך להתחיל מכח שהשעה עוברת או כדי לסמוך גאולה לתפלה דינא הכי [טור וד"ע]:

If one started praying [the Amidah] along with the prayer leader [i.e. when one did not recite a private Amidah first], when one arrives along with the prayer leader at "Nakdishakh" [the beginning of the Kedushah], one should recite the entire Kedushah along with [the prayer leader], word by word, just like [the prayer leader] is reciting [it]. And similarly, one should say with [the prayer leader] word by word the blessing of "HaEil HaKadosh and the blessing of "Shomeah Tefillah". Also one should focus one's attention so that when the prayer leader arrives at "Modim", one also arrives at "Modim" and at "ha-tov shim'kha u'lekha na'eh l'hodot" [i.e. the beginning and the end of the Hoda'ah blessing], so that one can bow along with the prayer leader at "Modim". Gloss: But from the outset, one should not start until after the prayer leader said Kedusha and "HaEil Hakadosh" unless it is required to beginning because the time [for the Amidah] is passing or in order to juxtapose Geula (i.e. the blessing of redemption that immediately precedes the Amidah) to [the Amidah] prayer, then the law is like this [Tur and his own opinion].

יחיד העומד בתפל' וכשיגיע למקום קדושה היו הצבור אומרים קדושה דסידרא אינו אומר קדוש עמהם שאין הקדושות שוות ונראה דה"ה אם היו הצבור אומרים כתר שאינו אומר עמהם קדוש אלא ישתוק ויכוין למה שאומרים דשומע כעונה: הגה וי"א דקדושת כתר דהיינו קדושת מוסף והיחיד מתפלל שחרית יוכל לומר עמהם דשניהם קדושת י"ח וקדושתן שוה וכן נ"ל עיקר [ולא פליג רשב"א אגאונים שהביא ב"י]:

[One who, as] an individual, is standing in prayer [i.e. reciting the Amidah] and when one reaches the place [where] Kedushah [is said], [finds that] the congregation was saying the Kedushah D'Sidrah [also known as the U'va Letzion prayer], one should not recite "Kadosh [Kadosh Kadosh...]" with them, because the [two] Kedushah [prayers] are not equivalent]. And it seems that the same applies if the congregation was saying "Keter" [the introduction to the Kedushah in the Musaf Amidah], [that] one should not say "Kadosh [Kadosh Kadosh]" with them; rather one should remain silent and concentrate on what they are saying, for [one will have fulfilled one's obligation based on the principle of] "one who heard is like one who responded". Gloss: And some say that [when the congregation is saying] the Kedushah of Keter, which refers to the Kedushah of Musaf, and the [one who is praying as an] individual is praying Shacharit, that one may say it with them because both of them are a Kedushah of a Shemonah Esrei [i.e. Amidah] and their Kedushah is equivalent. And this seems to me to be the principle. [And the Rashba does not debate against the Geonim that the Beit Yosef cites]