Greetingz...
Basically you'd do the following...
PHP Code:
if ($_POST["CheckBox"] == "A")
// Send email
if ($_POST["CheckBox"] == "B")
// Send email
if ($_POST["CheckBox"] == "C")
// Send email
"CheckBox" -> the name of the checkbox...
<INPUT NAME="CheckBox" VALUE="A">
Hope That Helps...