Thanks for the reply.
Yes I have tried the last option. I kept C1, C2, C3, B1, A1 in one group so that B1 and A1 can see all the cases that are assigned to the group. Then set the role for A1 and B1 as "GROUP" to all modules.
C1, C2, C3 have individual roles as owner. But however my problem is when C1 assigns a case to C2 , new page appears saying "You do not have access to this area. Contact your site administrator to obtain access." Even though this page shows like this, case is actually assign to C2. How to prevent this page from appearing?