Although it seems like the main mitzvah of hearing the shofar is the first set of 30 shofar blasts sounded in succession after the Shofar Blessing is recited, in fact the mitzvah is really to hear the shofar integrated with the Mussaf for Rosh Hashanah, therefore the central shofar blowing is actually the three breaks during Mussaf when the shofar is sounded.
This point is explained clearly in the Chayei Adam (142):
עיקר התקיעות הוא לתקוע על סדר הברכות כחוזר ה”ץ תפילת המוספים שתוקעים למלכיות ולזכרונות ולופרות ומדינא הוא לתקוע על כל ברכה תר”ת ולפי הספק שנסתפקו בתרועה היה לנו לתקוע על כל ברכה תר”ת פ”א ותש”ת פ”א ותר”ת פ”א וכן נוהגין במקצת מקומות אך כיון תקנו חז”ל לתקוע קודם מוסף והם נקראין תקיעות דמיושב ר”ל שעדיין הקהל יובין ולאעומדין בתפילה והטעם שתקנו כן לערבב השטן שלא יקטרג בתפילת המוספין ובתקיעותיהן
If we are required to hear Tekiah-Teruah-Tekiah repeated three times, it would seem that at each of these junctures we should hear all three possible variations, i.e. the same 30 sounds repeated three times during Mussaf — in conjunction with Malchuyos, Zichronos and Shofaros.
Yet the prevailing custom is to play only a single variation of Tekiah-Teruah-Tekiah each time. Why is that considered good enough?
First we have to understand the reason for the initial 30 shofar sounds, known as Tekios D’Meyushav. This custom is intended to confound Satan so that he cannot act as a Prosecutor during Mussaf and the Mussaf shofar blasts. For Satan is alarmed by the sound of the shofar, which is a reminder of the Great Shofar to be heard in the future Redemption.
And it shall comes to pass on that day that a Great Shofar shall be blown, nd they shall come who were lost in the land of Ashur, and the outcasts in the land of Egypt, and shall worship the Lord on the holy mountain of Jerusalem. (Yeshayahu 27:13)
Upon hearing the Great Shofar the Satan, i.e. death and the Evil Inclination, will be eradicated from the face of the Earth.
And He will destroy on this mountain, the covering that is cast over all the people, and the veil that is spread over all the nations. He iwll destroy death forever and the Lord G-d ill wipe away tears from all faces… (Yeshayahua 25:8).
Since in essence the congregation fulfills the mitzvah through those initial 30 blasts, Chazal did not want to impose an unnecessary burden on the congregation by requiring them to be played another three times.
Andy why was the “Shevarim-Teruah” variation chosen? Because in a way it is the safest bet. If the true Teruah is “Shevarim” you hear it; if the true Teruah is what we refer today as Teruah, you hear it. The only problem is that perhaps you are hearing an extra note, rather than the three notes in succession. But this is considered a minor enough concern that we avoid imposing a burden on the congregation.