Manage relationships (unique values needed)

Hello all,

I'm new to Power BI, and I'm trying to manage relationships between 3 tables -- "Customers", "Sales Orders", and "Cash Sales".

The Customers table includes a list of all current customers with a 3 month time period (Q3 2015).

Ex:

Customers      

Customer 123

Customer 456

Customer 789

The Sales Orders table includes the Sales Orders for each of those customers

SO#                       Customers      

SO-400                  Customer 123

SO-401                  Customer 456

SO-402                  Customer 789

To illustrate, I have a Sales Order with 3 distinct items, so in the Cash Sales table, it displays the same Sales Order number in three consecutive rows right next to 3 different item numbers.

Ex:

SO#               Item#          Customers            Date of Sale        

SO-401         9900            Customer 456       09/01/2015

SO-401         9921            Customer 456       09/01/2015

SO-401         9931            Customer 456       09/01/2015

I want to create a relationship for the "Customers" column in the Customers table to the same column in the Sales Orders table, which works perfectly. The problem I seem to have is when I try to create a relationship between the SO# column in the Sales Orders table and the Cash Sales table (due to the Cash Sales table having duplicate values for the SO#).

Is there any way to "roll up" the duplicate values in the SO# column so I can create the relationship with unique values?

Thank you in advance to anyone that can help! :)