GIRM paragraph 52
In the extraordinary rite (once the ordinary rite) the Kyrie is repeated nine times. Three is a perfection number symbolically, so to repeat each of the calls three times (3 x 3) lifts you up to nine which is a perfection number on steroids. We are asking for mercy in a symbolically perfected way, which should remind us to do so in our hearts.
In the ordinary form, the Kyrie Eleison (a left over from the Greek speaking days of the Church) is usually said six times with a “Call, response; call response; call, response.” It always follows the Penitential Rite unless it was woven into the Penitential Rite. By this I mean when the priest or deacon says something along the lines of, “You were sent to heal the contrite of heart, Kyrie Eleison (Lord have mercy.)
Before the new Missal came out, it seemed there was more leeway in the invocations. (You could make junk up.) So someone might say, “We are spineless vermin, Lord have mercy.” It appears to me that the new Missal is pretty adamant that we use approved texts.
Remember we said that words are chosen very carefully in the rubrics (directions for saying Mass)? It says here that this part of the Mass “should” be executed by everyone present. So, on a regular basis, everyone would be responding, “Lord have mercy, Christ have mercy, Lord have mercy.” That being said, “should” means that there are exceptions to this general rule. Say you had a fantasticarino, polyphonic and complicated Kyrie in Swahili by a visiting choir from Mars that only they are going to be able to sing while we listen and pray along, that’s Okay.
And finally, something new I learned but never plan on employing: although “usually” each acclamation is pronounced twice, “usually” being the operative term, it may be repeated any number of times depending upon certain circumstances such as the beauty and construction of the music or the use of various languages.
Lots of stuff to know for a relatively easy act no? But the rubricists are to the Mass what NASA is to space travel: prepare for every contingency!