Also, aside from what ernie stated, it looks like its on 2 different pages, but its not stated in the action. So either change the action to reflect the second page or put the php above the html.
$mess = "you have pressed A";
$mess = "you have pressed B";
$mess = "you have pressed C";
<form action="" action="post">
<input type ="submit" name ="Button" value = "A"/>
<input type ="submit" name ="Button" value = "B"/>
One other thing, that last print won't work. All you'll get is Array. If want to see what's contained in the POST array, you'll need to either run it through a loop, use print_r() or var_dump(). Including the page doesn't really do any good since you're starting on that page.