Dax Query optimization

I agree that measure looks pretty good, and would have written it the same way. How big is your Fact table and how many rows should that SUMMARIZE table have?  One thing you could try would be to filter it sequentially with FILTER(FILTER(FILTER. Not sure it will help much but easy to try.

Correct Total Sum =
SUMX (
    FILTER (
        FILTER (
            FILTER (
                ADDCOLUMNS (
                    SUMMARIZE ( Fact, dimA[Aname], dimB[BName] ),
                    "@MeasureA", [MeasureA]
                ),
                [@MeasureA] > 0
            ),
            [MeasureB] > 0
        ),
        [MeasureC] > 0
    ),
    [@MeasureA]
)

Pat