There is a trick to do this in SSAS MD - I call it "fictitious measure", please find explanation below.
Create a dumb measure in the measure group with A, B, C of your example always returning NULL. Aggregation function - sum, because you requested it to be sum in your question.
Define a SCOPE
on this measure like
SCOPE ([Dim Product].[Product].Members, [Measures].[Dumb Measure]);
This = [Measure].[B] / [Measure].[A] * [Measure].[C];
END SCOPE
This measure will be defined with the given formula on Dim Product
level and will be summed further on.
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