Output from Form with PHP IF statment

I can't tell why my php file is not running my script, and feel like there is a syntax error with me mixing html and php but don't know enough to tell where the issue is.

Below is an example of a basic html form I have created...

Choose any two of the following...

<form method="get" action="output.php" target="_blank">
<input type="checkbox" name="option1" value="5">Option 1
<input type="checkbox" name="option2" value="10">Option 2
<input type="checkbox" name="option3" value="15">Option 3
<input type="submit" value="Submit">
</form>

This form seems to work great. But when user is redirected to 'output.php' I'm getting an error. The code for the custom template looks something like this...

<?php Template Name: Output ?>
<html>
<head></head>
<body>
<?php
  if ($_GET['option1']+$_GET['option2']+$_GET['option3'] = '15') {
     echo "Good";}
  if ($_GET['option1']+$_GET['option2']+$_GET['option3'] = '20') {
     echo "Better";}
  if ($_GET['option1']+$_GET['option2']+$_GET['option3'] = '25') {
     echo "Best";}
}
?>
</body>
<footer></footer>
</html>

Is there an obvious syntax issue that I'm overlooking here?

Thanks in advance!