Zkuste
Select (Coalesce(x,0) + Coalesce(y,0) + Coalesce(z,0)) /
(Coalesce(x/x, 0) + Coalesce(y/y, 0) + Coalesce(z/z, 0))
nebo
Select (Coalesce(x,0) + Coalesce(y,0) + Coalesce(z,0)) /
(Case When x Is Null 0 Else 1 End +
Case When y Is Null 0 Else 1 End +
Case When z Is Null 0 Else 1 End)