Did you ever figure this out? What REST API call are you attempting? This link will outline what the particular error code means based on what call you were making: https://onlinehelp.tableau.com/current/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#API_Reference. Based on looking at the typical reason for 403s, they all appear related to permissions. Most of the explanations include the phrase "A non-administrator user called this method but doesn't have permission..."
Hope this helps!