If you just want to sum B1 across Sheet1 to Sheet3...
=SUM(Sheet1:Sheet3!B1)
If you just want to count numbers B1 houses across Sheet1 to Sheet3...
=COUNT(Sheet1:Sheet3!B1)
If you want to sum B1 when B1 > 0 (exluding negative values) across Sheet1
to Sheet3...
[1] If you have the morefunc.xll add-in installed:
For Conditional Summing:
=SUMPRODUCT(--(THREED(Sheet1:Sheet3!B1)>0),THREED(Sheet1:Sheet3!B1))
For Conditional Counting:
=COUNTIF.3D(Sheet1:Sheet3!B1,">0")
[2] If you need to do it without such an add-in:
For Conditional Summing:
=SUMPRODUCT(SUMIF(INDIRECT("'"&Sheets&"'!B1"),">0"))
For Conditional Counting:
=SUMPRODUCT(COUNTIF(INDIRECT("'"&Sheets&"'!B1"),">0"))
where Sheets refers to a range housing the relevant sheet names, that is,
Sheet1, Sheet2, and Sheet3.