"Ola" wrote...
Do you mean? :

=IF(OR(ISNUMBER(A3),ISNUMBER(A2),ISNUMBER(A1)), A3-A2+A1,"")
'AND' instead of 'OR' if all cells must be numbers.

Alternatively, 'OR' semantics:
=IF(COUNT(A1:A3)=1,A3-A2+A1,"")

'AND' semantice:
=IF(COUNT(A1:A3)=3,A3-A2+A1,"")