if you have separate measures then create a new calculated field sum(measure1) + sum(measure2) + sum(measure3)
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So you should be able to use
zn([Measure 1]) + zn([Measure 2]) + zn([Measure 3])
The ZN equates a NULL to Zero, which is needed when adding across measures (1 + NULL + 1 = NULL not 2). When nulls appear in a single column the NULLs won't get counted or added in a sum, but across measures we need to use the ZN
Hopefully that's it, but let me know if that doesn't solve it