1. When you set up your dorms in Booking.com Extranet and Expediacentral, you should remember these portals initially sold to sell "beds", but "entire rooms".

That is why, the logic of the portals works this way:

When guests select "1" in the window above, he will make the booking for "3 people" in HostelSystem. 

Therefore, this booking should be entered to the private room which can host 3 people or more.

2. That is why, it is critical to remember that when you set up dorms, you must set up "Maximum Occupancy" to "1":

When guest selects on the screen below "1", he wants to book "1 bed", if he selects "2", then he will book two beds.

In case if you have your "Maximum Occupancy" set up to another number, you are selling "1 bed" to multiple guests!

Guest books "1 bed", but he books place for e.g. "3 people for the price of one bed". 

3. This is how it should look on Expedia:

If you cannot see it in your Expediacentral, check what if you see while you search your property on Expedia.com:

Please note that this setting is critical in order to not receive overbookings and not to sell beds below its selling price.