You can use something like this:
var number A = SumValues(GetRecords("{3.LTE." & [Record ID#] & "}"), fidA);
var number B = SumValues(GetRecords("{3.LTE." & [Record ID#] & "}"), fidB);
var number C = SumValues(GetRecords("{3.LTE." & [Record ID#] & "}"), fidC);
$A + $B +$C​
Replace fidA, fidB, and fidC with the field ID for your Fields A, B, and C.
One Caveat with this approach, if the table is large or has several inter app relationships, using three formula queries in one field could degrade performance.
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Paul Peterson
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