Add this class to HTML if nth child in PHP block - php

Question

I have a set of case studies which vary in size. Like so:
| Large | Large |
|Small | Small | Small |
| Large | Large |
...
I'm trying to run a loop which will check if the case study is the first or second result pulled, if so, use .case-card--large CSS, else use the default .case-card CSS.
I'm unsure on how to use select classes if the loop condition is true?
In plain English: if first two results, use .case-card--large, then for the next three use .case-card, then again, use .case-card--large for the next two and so on.
Here's my loop so far (incomplete):
<?php
$counter = 1;
$loop = new WP_Query( array( 'post_type' => 'case-studies') );
while ( $loop->have_posts() ) : $loop->the_post();
if ($counter % 3 == 1){
echo 'alpha';
} else {
echo 'omega';
}
$counter++ ;
endwhile
?>
Output: alpha omega omega alpha omega, when I need it to be alpha alpha omega omega omega.
The div and CSS:
.case-card--large {
width: 50%;
border: 1px solid red;
}
.case-card {
text-align: center;
padding: 40px;
width: 33%;
}
<div class="case-card case-card--large" <?php echo tp_bg_image($background, 'full' ); ?>
<div class="wrapper">
<p>ID:
<?php the_ID(); ?>
</p>
<h1>
<?php echo $heading; ?>
</h1>
<p>
<?php echo $subheading; ?>
</p>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#case-study-video">
Click me
</button>
</div>
</div>
Is a loop the right away to go about this?