SSAS Calculated measure with SCOPE.
I don't have your SSAS database so I can't test the syntax, but I believe you would need to write your MDX as follows (notice the THIS= addition):

SCOPE ([Measures].[Total]); 
THIS = SUM([Data Source].[Interpolation Method].&amp;[3],[Measures].[Total]); 
END SCOPE; 


See the documentation for syntax details.