FluentFormCheckbox_select_all_options.php

add_filter('fluentform/validate_input_item_input_checkbox', function($validation, $field, $formData, $fields, $form) {
   // Do your stuff here
   // 
   
	$formId = $form->id;
	
	if($formId != 6) return $validation; // Change the form id here
   
	
	$count = count($formData['checkbox']); //replace checkbox with the name attribute of the field
	
	
	if($count<3) return 'Please select all the options'; //update your message and replace 3 with total option in the checkbox

   return $validation;
}, 10, 5);