Excel 2003 :: Conditional Format Top / Mid / Bottom 33% Of Cells But Ignoring Blank Cells

I am trying to conditionally format the top middle and bottom thirds of a range of data. Problem is, that the range needs to be flexible as sometimes there may be a maximum of 36 cells with data, but sometimes there may be less (so there are blank cells in the range that need not be counted). The methods I have tried always include the blank cells, and so it is not equally formatting the thirds (as it includes the blanks cells as part of the bottom data)....

Here are the 2 methods Ive tried so far using excel 2003)
Top 34%:
=IF(INT(COUNT($D$3:$D$38)*34%)>0,LARGE($D$3:$D$38,INT(COUNT($D$3:$D
$38)*34%)),MAX( $D$3:$D$38))0,LARGE($D$3:$D$38,INT(COUNT($D$3:$D
$38)*67%)),MAX( $D$3:$D$38))0,LARGE($D$3:$D$38,INT(COUNT($D$3:$D
$38)*100%)),MAX( $D$3:$D$38))