get_row and Select all

I have a SQL statement that works like this:

$qnm = $wpdb->get_row($wpdb->prepare(“SELECT col1,col2 FROM ml_table WHERE col3 = $somevariable”));
I return results like $qnm->col1 and get a value

But it does NOT work like this:

$qnm = $wpdb->get_row($wpdb->prepare(“SELECT * FROM ml_table WHERE col3 = $somevariable”));
I return results like $qnm->col1 and do NOT get a value

Why does the Select specific columns work, but Select * does not? I’ve tried reading through documentation but can’t find anything. Thanks in advance!