Usually it happens out of necessity and not from a pedagogical point of view. Too few students to put together a group 2, not enough children for a group 3. Then together. The combination group 2/3 can cause the teacher a lot of headaches. Because: how do you teach children to read in a class full of playing preschoolers? It can be done and sometimes even has major advantages.
Within a theme you can search for learning activities that match the development of the children. This is how you provide tailor-made education
If you want to go through the methods for group 3 step by step, for example, there is not enough time for group 2. But a method is not the ultimate solution. Get that from the methods you need, but otherwise draw your own plan. Of course, the students must be monitored. They shouldn't get rid of it. But usually the children are so enthusiastic about a theme that they don't. Because they bear responsibility for their own part within that theme, they are fully committed.
Teaching time is very precious. Especially in a combination group. You have to organize well in a combination group. Make the group aware that, for example, they are not allowed to enter the house corner when group 3 is given instructions, because a lot of noise is made there. They are also not allowed to disturb the master or teacher during the instruction time.
Do not hesitate to knock on the door of your colleagues if you encounter difficulties. When a school puts together a group 2/3, it is a decision of the team. So as a team, you also have to deal with any problems.
Take a look at a combination group at another school. What is the approach there? You can learn how you do and maybe don't want to do it, and thus arrive at your own interpretation.
When working with tasks, preschoolers are very independent. In many schools this is quashed by working with methods in group 3. Avoid that. Through these combination groups you continue to appeal to the independence of students, also in group 3. This will benefit them for the rest of their lives.
At the beginning of the year, that appeal to self-reliance can be a bit too much to ask. Because learning to read is new to group 3 children, they are often still a bit dependent and need a lot of attention. The first weeks of the year, therefore, a lot of attention is paid to group 3, but you will reap the benefits for the rest of the year. If possible, try to get help from a teaching assistant or intern during those first weeks.
There are always children from group 2 who listen with half an ear to the instruction for group 3. If they want to participate in group 3, that is possible. In the beginning, group 2 often participates very enthusiastically with group 3, but after a few weeks the news is over and the children disappear into the corners again. The fact is that children in this environment are more stimulated.
The transition from group 2 to group 3 is a stumbling block for many children. At first they were allowed to play independently, now they have to learn to read in class. This is much more gradual in a combination group and that is nice. Both groups can learn a lot from each other. Tailor-made education is good for every child and this ongoing development means that the transition to group 3 is no longer so scary.