To calculate the average of several variables, you need to know what you want to do when you have DK values. When you have at least one DK/NA value, in Askia Analyse  or in AskiaVista, the sum of variables will be equal to DK:

DK/NA + Var2 + Var3 = DK

Var1 + DK/NA + Var3 = DK

Var1 + Var2 + DK/NA  = DK

So, to handle the interview with a DK response, you have two possibilities:

1. Removing the interview

If you have at least one DK/NA value, you want to remove the full interview from the average calculation. (Calculated variable Numeric by script):

Dim arrQs As QuestionArray
arrQs.Push(^Q1^).Push(^Q2^).Push(^Q3^)
Dim arrVals = {}

Dim i 
For i = 1 to arrQs.Count
  If arrQs[i].Value <> DK Then 
    arrVals.Push(arrQs[i].Value.ToNumber())
  Else 
    arrVals = {}
    Exit for
  Endif
Next i

If arrVals has {DK} Then
  Return DK
Else 
  Return arrVals.Sum()/arrVals.Count
EndIf

2. Keep the interview and ignore the variable 

You have at least one DK/NA value, you exclude the variable from the calculation but you keep the interview for other valid variables (calculated variable numeric by script):

Dim arrQs As QuestionArray
arrQs.Push(^Q1^).Push(^Q2^).Push(^Q3^)
Dim arrVals = {}

Dim i 
For i = 1 to arrQs.Count
  If arrQs[i].Value <> NR Then 
    arrVals.Push(arrQs[i].Value.ToNumber())
  Endif
Next i

Return arrVals.Sum()/arrVals.Count

You can also highlight all three variables in the Question tree > Create loop. The resulting looped variables can give you some additional flexibility to analyse the aggregated data of the source variables.