Hierarchical Structure
One way to define your tables:

Users: user_id (primary key), name, etc

Group_A: user_id, group_a_id, group_a_name

Group_B: group_a_id, group_b_id, group_b_name

Group_C: group_b_id, group_c_id, group_c_name

Items: group_c_id, item_id, item_name, details