Try this...
Sheet1
<TABLE style="PADDING-RIGHT: 2pt; PADDING-LEFT: 2pt; FONT-SIZE: 10pt; FONT-FAMILY: Verdana,Arial; BACKGROUND-COLOR: #ffffff" cellSpacing=0 cellPadding=0 border=0><COLGROUP><COL style="FONT-WEIGHT: bold; WIDTH: 30px"><COL style="WIDTH: 51px"></COLGROUP><TBODY><TR style="FONT-WEIGHT: bold; FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center"><TD></TD><TD>A</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">1</TD><TD style="BORDER-RIGHT: #000000 1px solid; BORDER-TOP: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-BOTTOM: #000000 1px solid">Header</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">2</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data1</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">3</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data1</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">4</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data2</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">5</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data2</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">6</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #99ccff">data3</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">7</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #99ccff">data3</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">8</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data4</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">9</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data4</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">10</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data4</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">11</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data5</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">12</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data5</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">13</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data5</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">14</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #99ccff">data6</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">15</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #99ccff">data6</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">16</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data7</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">17</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ccffcc">data7</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">18</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data8</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">19</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #ffff99">data8</TD></TR><TR style="HEIGHT: 17px"><TD style="FONT-SIZE: 8pt; BACKGROUND-COLOR: #cacaca; TEXT-ALIGN: center">20</TD><TD style="BORDER-TOP-WIDTH: 1px; BORDER-RIGHT: #000000 1px solid; BORDER-LEFT: #000000 1px solid; BORDER-TOP-COLOR: #000000; BORDER-BOTTOM: #000000 1px solid; BACKGROUND-COLOR: #99ccff">data9</TD></TR></TBODY></TABLE>
Select the *entire* range A2:A20 starting from cell A2. Cell A2 will be the active cell. The active cell is the one cell in the selected range that is not shaded. The formula will be relative to the active cell.
Goto the menu Format>Conditional Formatting
Condition 1
Select the Formula Is option
Enter this formula in the box on the right:
=MOD(ROUND(SUMPRODUCT((A$2:A2<>"")/COUNTIF(A$2:A2,A$2:A2)),0),3)=1
Click the Format button
Select the Patterns tab
Select the desired fill color
OK
Click the Add button
Condition 2
Select the Formula Is option
Enter this formula in the box on the right:
=MOD(ROUND(SUMPRODUCT((A$2:A2<>"")/COUNTIF(A$2:A2,A$2:A2)),0),3)=2
Click the Format button
Select the Patterns tab
Select the desired fill color
OK
Click the Add button
Condition 3
Select the Formula Is option
Enter this formula in the box on the right:
=MOD(ROUND(SUMPRODUCT((A$2:A2<>"")/COUNTIF(A$2:A2,A$2:A2)),0),3)=0
Click the Format button
Select the Patterns tab
Select the desired fill color
OK out