It is not possible to create subgroups automatically but you can do it manually.
Create a group division with two groups, for example Group A and Group B.
Make a random grouping of the participants and click on Save.
Create a new group division with three groups, for example A1, A2, A3, B1, B2, B3.
Open Participant grouping for the new group division.
Click on Displaying the list's columns and select the column Groups.
You will now see which students belong to Group A and Group B. Divide all the students in Group A so that they belong to either group A1, A2 or A3 and divide all the students in Group B som that they belong to either group B1, B2 or B3.