For example if I have 3 collaborative tasks to give to the students, which one is better either same student in the groups working the 3 tasks together all the time or change the group members each time when giving new different collaborative task?
Cooperative learning is an organized and structured way to use small groups to enhance student learning and interdependence. There are many benefits of using cooperative learning in the classroom: Students enjoy it and are more motivated. ... Students learn to work together.
In my opinion, changing the group members enhances creativity and promotes competition. In my practical experience i have observed this. Hope this will work for you too.