You can't have different height cells in the same row.
Try this.
<TABLE WIDTH="400" HEIGHT="150" BORDER="0" CELLPADDING="0" CELLSPACING="0">
<TR>
<TD WIDTH="152" HEIGHT="120">LEFT CELL</TD>
<TD WIDTH="310" HEIGHT="240" ROWSPAN="2">MIDDLE CELL</TD>
<TD WIDTH="152" HEIGHT="120">RIGHT CELL</TD>
</TR>
<TR>
<TD HEIGHT="28">unused cell</TD>
<TD>unused cell</TD>
</TR>
</TABLE>
Or...
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0">
<TR>
<TD WIDTH="152" HEIGHT="240" VALIGN="MIDDLE">
<CENTER>
LEFT
</CENTER></TD>
<TD WIDTH="310" HEIGHT="240" VALIGN="MIDDLE">
<CENTER>
MIDDLE
</CENTER></TD>
<TD WIDTH="152" HEIGHT="240" VALIGN="MIDDLE">
<CENTER>
RIGHT
</CENTER></TD>
</TR>
</TABLE>