A custom object with two master-detail relationships. Using a custom junction object, you can model a “many-to-many” relationship between two objects.
Add reference field to Page Layouts > Next
Add custom related list > Save (Master-detail relationship field is created)
Step 1. Create Object
Create > Objects > New Custom Object
Do not launch Custom Tab Wizard after saving this custom object.
Create two master detail relationship
Custom Fields & Relationships > New
Select Master Detail Relationship
Related To > Account (Choose the related object)
Enter Field Name
Sharing Settings: Sharing access that users must have to a master record in order to create, edit, or delete its associated detail records.
Field-Level Security for reference field (Cannot be changed).
Similarly create second master-detail relationship field on the object.
Account Object > Select Record (Related list will be created on the record)