Referring Foreign key to primary keys across multiple tables

Table 2 and table 3 are having many-to-many relationship, for example:
1. a teacher (table 1) may teach more than one class (table 2) and a class may be taught by more than one teacher.
2. a customer (table 1) may buy any number of products (table 2) and a product may be bought by any number of customers.
To link these 2 tables, you need a third table (table 1) to pull them together. This table will have at least 2 fields, one each linking to the primary keys of the respective tables. One example:

Table Name      Columns
tbl_customer    cust_id (primary key)
                cust_name 
tbl_product
                product_id (primary key)
                product_name
                price
tbl_order
                order_id (primary key)
                cust_id (foreign key to tbl_customer)
                product_id (foreign key to tbl_product)

To find a particular order detail, you have to JOIN them together:

select order_id, cust_name, product_name, price from tbl_customer c join tbl_order o on c.cust_id = o.cust_id join tbl_product p on p.product_id = o.product_id where order_id=2

Reference:
1. http://www.datanamic.com/support/lt-dez005-introduction-db-modeling.html[^]
2. Visual Representation of SQL Joins[^]

key's don't actually matter one jot for joins, they are used for enforcing constraints i.e. if you delete a customer then the constraint can delete all the orders that contain that customer_id.

can join anything to anything though, just learn the differences between inner and outer joins.