What if we have all the product available in the building? What if today we write an order that isn't going to be delivered for 3 months and tomorrow we write an order for immediate shipment? Will it still reserve the product that isn't shipping for 3 months before it reserves it for the immediate shipment?

The reservation logic works the same whether you have product available or do not have product available. The earliest delivery will be the first to be reserved, the one for three months later will only be reserved when all other orders before it have been reserved. If you initially have a delivery for three months in the future that has stock reserved, and then AFTER that you enter orders for earlier delivery, you can unreserve product from the later order to make it available for the earlier orders - otherwise the reservation holds and the earlier orders won't be allocated a reservation unless there is additional product available. Some users choose to batch "Unreserve" all orders periodically so that products reserved for orders later in the month (where those reservations were placed before other orders for earlier in the month) can be unreserved and made available for sales based on the order of delivery dates. The ACTION Menu once you have selected multiple Delivery Orders is where you will find the UNRESERVE option to work on multiple Deliveries at once.