Hello Guillaume,
as far as I know it's not possible to 'sell' more items without requiring the user to 'buy' them (at least from the front-end): when you add 3 times a "Pack of 3" to the cart, it would decrease the stock level with 3.
However using the backend (Shopify API) you can watch for sales and decrease the stock level whenever a Product is sold. It should be possible to develop this App if you can't find a suitable App in the App store.
HJ