Students chatter happily when they hear that we are going to use the blocks again (Take Time for Tens: Acts 1 & 2). One child, who had been in the more advanced group yesterday, says, “Can I use the blocks today instead?” I ask why and he replies, “Because it’s fun.” I say yes. Other kids cheer. A few kids say, “This isn’t like math because it’s fun.” I, of course, say, “You mean math isn’t usually fun?” They sheepishly respond, “Well, no, it IS, but this is like playing.”
So prior to the start of the lesson, I ask them to think, in their heads (versus sharing!) about why they like working with the blocks, and about what it is, specifically, that they find helpful. That’s a hard idea to articulate so after a “30 second think” we move on. I sometimes do not have them share aloud. I think it is okay to prompt students to think on a topic and then let them keep that idea in their head for a while. Everything doesn’t have to be verbalized to be of value!
Students are already familiar with the process of using the cubes from the two previous lessons so I only need to repeat the obvious once, "Keep all cubes off the mat that aren't part of the current equation, don't play with the blocks  they are tools!"
I only go through four examples before starting the guided practice, because everyone but one child is ready. I work with the one individual separately once the class is started on the independent practice.
128 + 136 = 
122 + 134 = 
175 + 196 = 
125 + 185 = 
I prepare bags of 30 tens (longs), 30 ones and 3 hundreds (flats) for each student prior to the lesson. This is a great task for student helpers.
Students come to the carpet with their whiteboards or their math journals.
We work through the following equations together.
136 + 68 = 
165 + 119= 
122 + 116=

109 + 199 = 
118 + 137= 
184 + 187 = 
I call on students randomly (not raised hands) to give me each step of the process of addition with regrouping. When students use imprecise language I restate using math vocabulary (MP6  Attend to precision).
Example:
Student: "You move this one, over here, to over here, and then get one of these."
Teacher: “You move a one from the one’s place in 184 to the one’s place in 189 and then you have 10 tens in the one’s place in 189 so you can exchange those ten ones for a ten, and now there are 9 tens and 0 ones in the one’s place and you have 190."
As students work through the each equation, I monitor carefully to insure the students are staying with me and not skipping or misrepresenting steps.
For the independent practice, I have students record the equation changes and answers in their journal but you could also use the Happy Hundreds Student SelfAssessment. This resource has a variety of regrouping problems to test different skills.
There are 2 problems that don't require regrouping, and I always have several children come over and tell me that they can’t do these problems. In 3rd grade, they are already trained to stay within the perceived constraints of an assignment. Of course they can still solve the problems that don't require regrouping! Also, they think it's a mistake that a slightly harder problem appears earlier on instead of at the end. To which I say, "Why is that a mistake?" These assumptions about math assignments are a different way to work with Mathematical Practice 6  Attend to precision. If they slow down and think through this through, they will realize they are jumping to false conclusions. Their logic is flawed. This is an important awareness.
I continue to call some students over for 1:1 assessments but I also monitor some children at their desks if they were successful with the 2digit regrouping and definitely ready to move on. Here a student explains his thinking as he models 3 digit addition using place value blocks.
I have students fill out this selfassessment and then I look at what they wrote. I'll use this, in addition to their work on paper and my notes, when planning flexible groups for the next lesson.