The simple answer (I don't know if there's a more elegant one) would be:
Totals: Iif(IsNull[field1],0,[field1] + Iif(IsNull[field2],0,[field2] + Iif(IsNull[field2],0,[field2];
This *should* work but it's off the top of my head. Look in VBA's help under 'iif' for its correct syntax if not...