These hotels need to be booked in advance, because there are generally many hotels in Beijing, and it is not easy to find vacancies.
Because you will stay for three days, and these three days should be relatively mobile, so you'd better decide which three nights to stay according to your itinerary. You can decide where to go for these three days, and then call to book the nearest and fastest hotel there, or you can check it online.
It is very important to arrange food and accommodation reasonably according to the itinerary. Be sure to arrange your itinerary before you go. If you are fully prepared, you will have a good time and won't miss the sights.
Chain hotels are about that price. You can apply for membership and enjoy preferential treatment.