Entity Relationship Model - double one to many ERD



A customer submits one or many  leads.
A lead belongs to one customer.
A lead is a request or an inquiry.
A lead belongs to one manufacturer.
A manufacturer has one or many leads.  



Tables  

Only focusing on manufacturer and lead I set up the tables like below.  

manufacturer            lead
--------------          ---------------
MId                     LId
name                    MId
description             date
...                     ... 


Adding customer table I add CustomerId to lead table.

manufacturer            lead                customer
--------------          ---------------     ---------------
MId                     LId                 CId
name                    MId                 phone
description             CId                 ...
...                     date   
                        ...




Issue:

With all three entity types, is there a benefit to have an additional linking table instead adding ManufacturerId(MId) and CustomerId(CId)  as column to Lead table? Does this offend any best practice? Or should I even include two separated linking tables? Thank you for Explanation and 'further reading links'.
Your LEAD table is an intersection between MANUFACTURER and CUSTOMER.  That is a perfectly legitimate design.  Intersection tables don't have to be pure intersections.  They can have other attributes in their own right (such as your date or type).