Things will not always go the way you expect. There were big differences between the three group discussions in terms of their activity and this sort of variability is a major difficulty when designing a course. You have to build in the flexibility and adaptability to cope with this sort of eventuality. What is most important is not losing student motivation, or making students feel left-out. When they lose the desire to participate then you have lost the battle.
There are a few possible solutions I was thinking about if you find a group is not participating as expected.
Dummy students: create accounts for fake students that can be used to make sure there are active discussions on the course. These dummy accounts can be used to seed discussion groups and to make sure that important questions are asked.
Give students very clearly defined roles: One of the weaknesses of online learning can be giving students too much freedom (Clark and Mayer). Students can be not very discerning or selective and they can be over-whelmed by too much choice. Three options is the most before people start to get confused (Gigeranzer).
Make sure that interactions are assessed: If you are going to have a task it has to be linked to assessment to make sure that students will participate.