Hi Andee,
The sum function expects a single expression to be summed over all cases. Here’s the documentation:
| sum(number, filter) | Returns the sum of its argument evaluated for every case. |
sum(grade_point) computes the sum of the grade points sum(Mass, Height>60) returns the sum of the masses for heights taller than 60 |
The strange results you get from sum(V1,V2,V3) are the result of summing the values of V1 over all cases for which V2 look true, totally ignoring V3.
V1+V2+V3 is the best way to get the sum of the values for the three attributes for each case.
Hope this helps,
Bill