As I understood in the comments, the question is about those daven Mincha, Kabolas Shabbos, and Maariv at the same time throughout the summer without regard to whether Mincha and Maariv are on separate sides of Plag. Many times it comes out that all 3 Tefillos can be between plag and Shkia, so why is there no issue of Tarti Dsasri.
The Mishna Berura in 233:11 brings down a shita that says this is permissible if it will be hard to gather a minyan again.
The Aruch Hashulchan 233:10 explains why this shita holds it is not a problem, he says, since Maariv is representing the Hekter Chalavim and Eivarim which were able to be done by day as well, therefore it is permissible to daven Maariv by day if needed.
מיהו יש נוהגים גם בזה, מפני שקשה לאסוף הציבור עוד פעם (מג"א סק"ז), ויתבאר בסימן רל"ה. ואפשר לומר הטעם כיון דתפלת ערבית נתקן נגד הקטרת איברים, והם כשרים ביום ובלילה, ושם יתבאר בסייעתא דשמיא.
The Shraga Hameir 5:21 strongly defends the minhag on Erev Shabbos to not be makpid on Plag, and offers an explanation that by being Mekabel Shabbos it effectively changes the day and allows for Maariv to be considered being davened on the next day. Obviously this would only work for Erev Shabbos but not on a regular weekday.