I have a queen bed with wooden head and foot boards and two(2) metal rails that are L shaped. The mattress sits on the horizontal portion of the rail quite snug/tight. Why were three wooden slat shipped with the bed? Are they necessary?

I think they are to support the middle part of the mattress.

I would think that the boards/slats are to be placed under the box springs equidistant from each other and going from the left to the right side of the bed while being placed on the metal frame. They will then support the middle of the mattress and box springs.

Many mattress companies have warranties to dictate whether a mattress set requires supports. For any bed larger than a queen, slats are required to adhere to the warranty. Some bedding manufacturers send supports with their products as a selling point. Other manufacturers do not send any at all, and consumers are required to make a separate purchase.