You'll want to create a query instead of a third table. We'll need more information on your database design to give you more specific help. If you can post a screenshot of your table relationship window that would be helpful.
Google and youtube "Access Queries" will return great general resources on how to use queries. Your query would look something like
Code:
SELECT Products.Base_Price * (1+Commissions.Commission_Rate) AS Total_Price FROM Products INNER JOIN Commissions ON Products.ProductID = Commissions.ProductID
Obviously I made up all the field names and made assumptions, but that should give you an idea. If the above looks like gibberish then you should definitely study some access videos on youtube or go through some tutorials.