When applying for available apartments, make sure you have all the requested documents you need for the signing process; passport/ID-card, registered student union membership, letter of acceptance/certificate of registration.
Three is the maximum amount of bookings you can have at the same time. Since you can’t rank your booking, make sure you only book housing that you really are interested in. The person with the most credit days out of the booking, will get the offer to sign the contract. Even if you have the most credit days for each of your bookings, you will only get one offer.