You may try to use: http://validator.w3.org/#validate_by_input enter your code there and you'll see a lot of 11 errors and 2 warnings. Your heading goes in the body not in your head.
This alone removes six of your errors (errors are still present, you need to run your code into the above link).
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>myTitle</title>
<meta charset = "UTF-8">
<link rel = "stylesheet" type = "text/css" href = "cssFile.css" >
</head>
<body>
<h3 class = "Heading">myHeading</h3>
</body>
</html>