Checkout: Action – checkout_process_order_X
checkout_process_order_X action hook runs after the billing step of checkout, when the customer has chosen the payment method to pay with.
X in the hook name is replaced with the slug of your payment method. So if the slug of your payment method is “mypmethod”, then the hook will be
add_action('checkout_process_order_mypmethod', 'process_order', 10, 3);
$order– An Object with all the data of the order
$user– An Object with all the data of the user (if available, logged in users only)
$params– Additional parameters which may be passed through