Now, we do not utilize the 3 period lesson as much in levels 2 and 3 - just when we are working with a brand new child at age 6 or 7.
Three period lessons are used to provide vocabulary in a very focused manner. Starting with 3 objects, providing the names (and functions for the older child); play little games with the objects, utilize them, manipulate them - the child knows them by you saying the name; finally the child can label the objects himself. As the child learns the names, more objects can be added.
Within both CGS and Montessori, there is an over-arching concept, beyond the three period lesson, that the child will have EXPERIENCE first. We don't just toss some items at the child and say "here are their names, now let's learn them" - no, it is all done in context of experience.
At elementary we can give experience and language at the same time; at primary we want experience first, then language so it becomes meaningful.
My Montessori experience is that the three period lesson is done in one sitting. If a child is truly stuck in one of the periods, then we say, "Thank you for joining me in this work today; we'll have more another time!" and we come back to it when the child is ready and refreshed. But for the most part, if a child "gets it", we move on and we might add additional objects at that sitting, if they are ready. This applies to names of objects, comparative qualities and superlative qualities.
CGS seems pretty adamant that the three-period lesson for one set of items is to potentially be dragged out over 2-3 years. I LOVE CGS. My concern here is that going too slow violates the child's dignity and intelligence. We are telling them, "You can't possibly learn these names in one fun little presentation." Again, some children DO need more time and we should honor that; but over and over again I see catechists who have a 3rd year child who still has not "completed" the 3-period-lesson for the basic articles of the Mass. By slowing down too much, we actually bring the child to a stand-still. Ouch!
In the first period, we provide the names while laying out the items. We then use the names several times ourselves as we re-name the items, replace them, etc. Many times the catechist will then invite the children to do a portion of the 2nd period, asking them to put away or take out particular items by name. But never in that first sitting have I seen a non-Montessori trained catechist even attempt a 3rd period in the first sitting, even for children who are eager-eager-eager to say those words! I'm not saying it doesn't happen - just sharing my own experience.
Perhaps it is that CGS is trying to give that experience first, since the children do not normally get to handle the objects of the Mass. Also, CGS is not about tests and quizzes; it is about being in relationship and developing that relationship; allowing the Holy Spirit to guide. This does not explain the extreme slowness of the process when this is nourishing food for the children. It happens with the geography work, other liturgical work and other areas as well. The three period lesson is not a test - it is a tool for the child to recognize what he already knows, to be able to verbalize what he already knows, to honor his development and intelligence, and thereby the dignity of his inner soul.
The other day, I had a conversation with a highly intelligent young lady (9 years old; has had a good deal of previous atrium experience) who was laying out the articles of the Mass and meditating upon the presence of Christ in the Eucharist. She had a green stole over one shoulder that a much younger child had placed there. During our conversation, I asked her about the item on her shoulder. She said, "This is after the feast." "Ah, I said - and what is it exactly?" "It is after the feast. Oh, it is Ordinary Time." She was saying the two phrases as if they were the name of the object, rather than what the object represented. That would be like saying a statue of Jesus IS Jesus. It is not that the mis-labeled stole leads directly to idolatry, but I am quite concerned that the mentality is set up towards that line of thinking, if we do not provide nomenclature in a timely and appropriate fashion for the young child.
Later, I gathered a group of 3rd and 4th graders and we did the 3 period naming lessons on the vestments of the priest. At this age, I could give a lot more detail on function. And we ended with the 3rd period, which was a bit of a stumbling block but they got it.
One boy reflected, "No one has ever asked me to actually SAY the names of these items myself without telling me the name right before. I like that. I actually know it now."