Adding to a plugin's options

Hi all – having bit of trouble adding an option to reduce stock on a product within a deposit login (woo commerce deposits). Currently the options as standard are: ‘deposit’ or ‘full payment’ – I wish to also add ‘both deposit or full payment’ so stock would be reduced if either deposit or full payment is taken.

Help, much appreciated!

Updated function#1:

public function payment_complete_reduce_order_stock($reduce, $order_id) {
    $order = wc_get_order($order_id);
    $status = $order->get_status();
    $reduce_on = get_option('wc_deposits_reduce_stock', 'full');

    if ($status === 'partially-paid' && $reduce_on === 'full') {
      $reduce = false;
    }
    else if ($status !== 'partially-paid' && $reduce_on === 'deposit') {
      $reduce = false;
    } //added in...
   else if ($status !== 'partially-paid' && $reduce_on === 'both' || $status == 'partially-paid' && $reduce_on === 'both'){
      $reduce = false;
    }

Updated function#2:

'deposits_stock' => array(
          'name'  => __('Reduce Stocks On', 'woocommerce-deposits'),
          'type'  => 'radio',
          'desc'  => __('Choose when to reduce stocks.', 'woocommerce-deposits'),
          'id'    => 'wc_deposits_reduce_stock',
          'options' => array(
            'deposit' => __('Deposit Payment', 'woocommerce-deposits'),
            'full'    => __('Full Payment', 'woocommerce-deposits'),
            // added in...
	    'both'    => __('Both Full Payment or Deposit', 'woocommerce-deposits')
          ),
          'default' => 'full'
        ),

This is where I think I fall down:

Original ‘Save all settings’ function:

$reduce_stock = isset($_POST['wc_deposits_reduce_stock']) ?
      ($_POST['wc_deposits_reduce_stock'] === 'deposit' ? 'deposit' : 'full') : 'full';

I’m not sure how to add in the additional option, this doesn’t work!:

$reduce_stock = isset($_POST['wc_deposits_reduce_stock']) ?
      ($_POST['wc_deposits_reduce_stock'] === 'deposit' ? 'deposit' : 'full' ? 'both' : 'full' ) : 'full';