Many to Many relationship is the best approach. In a modern database, this should not make any performance issues even if you have thousands of users.

However if you are sure that you have only 3 roles in the system and its not going to grow to 4 or 5, you can also follow the approach of giving a numeric value to indicate the roles similar to the chmod command in unix. client -1, recipient - 2, administrator 4. A total value of 7 indicates all 3 roles, 3 indicates client+receipient, 6 indicates recipent+admin, 4 indicates only admin etc.