I think we have a bug here. I just tried the following:
- open Excel (2016 preview) from scratch;
- in E1, enter formula =ISBLANK($B$3:$B$5)
- copy the formula from E1 down and across over range E1:L10
Because I locked the references, the formula is exactly the same in all cells E1:L10, but the results in rows 3, 4, 5 are TRUE, the rest are FALSE.
At no point did I even
look at cells B3:B5, never mind put anything in them!
edit: I thought it might help to enter the formula as an
array formula, but that didn't work properly either. It looks to me as if ISBLANK just doesn't work on a range.
This works: =(COUNTBLANK($B$3:$B$5)=3) is TRUE if all 3 cells are blank, FALSE if one or more has something in it. Putting that in to your formula, we have
=IF(COUNTBLANK(B3:B5)=3,"",SUM(B3:B5))
PPS: Excel actually has an option to suppress zero values. In recent versions it's under File / Options, Advanced section. "Show a zero in cells that have zero value" is normally checked, uncheck it to suppress zeros.