Would this not be where you calculate a total as sum(measure1) + sum(measure2) + sum(measure3).
and then create calculations of Measure1%ofTotal = sum(measure1) / total
measure2%ofTotal = sum(measure2)/total
Create 3 Calculated Fields as per your explanation in the Question's post.
Then add those 3 & Dimensions to the view.
1) Name: Total
Syntax: sum([Sales]) + sum([Orders]) + Sum([Return_Amount])
Syntax: sum([Sales])/ [Total]
Note: Total is the Calculation which was created in the first Step.
Syntax: sum([Sales])/ [Total]])
Syntax: sum([Return_Amount])/ [Total]])