Do something depending of the custom field's value

I have custom fields with the value of 1,2,3,4,5. I want to output something in my .php file depending on which value the custom field are holding. Lets say i only want to output something, if and only the value is "3".

 if ( ! empty( $custom_field ) ) {
        echo do_action( 'do_something' );
    }

This code determine only if the custom field is empty or not. How do i also get the value as an argument?