I haven't posted in awhile so I thought I would put something up before Shabbos.
Last week I discussed the issue of trei kalei lo mishtamai. The gemara learns it out from the fact that zachor and shamor were said at the same time and this is called a neis because it is impossible to hear 2 things at once. The gemara says the only exception is where you ar ehearing something that is chaviv like megilla or tekias shofar. There is a machlokes Rishonim why chaviv works. The Ritva says you are able to concentrate on one voice. The Ran says you hear both voices but you can understand both of them. A nafka mina is found in the Biur Halacha in Hilchos Shofar. If you hear 3 kolos from 3 people at the same time have you heard 3 kolos or 9 kolos? According to the Ritva we would say you only heard 3 but according to teh RAn you could say you heard all 9. The Mishna Brerura is machmir like the Ritva.
I famous issue that comes up based on this rule is how many people should say kaddish at one time. The Binyan Tzion and Chasam Sofer felt only one person should say kaddish because of trei kalei lo mishtamai. The Chasam Sofer held the point of saying kaddish was that it was a zechus for the niftar to have so many people answering amein. Since it is an issue of hearing kaddish the klal of trei kalei lo mishtamai applies here. Rav Ya'akov Emden in his siddur brings the minhag of sefardim to have everyone say kaddish. The Binyan Tzion argues that since sefardim daven togetehr it is easy to hear what everyone is saying and the rule of trei kalei lo mishtamai would not apply. Ashkinazim are not so unified and if everyone would say kaddish you wouldn't be able to understand anyone.
One could also argue that the ma'aleh of kaddish is in saying the tefilla regardless if people hear it so again the rule of trei kalei lo mishtamai wouldn't apply.
The Tzitz Eliezer seems to like the minhag of one person saying kaddish but he won't say it is wrong to do otherwise. Other poskim suggest everyone saying kaddish come to one area to minimize the problem.