Where to buy hong kong airport express tickets from Macau

We will be coming from Macau and will stay at a hotel in Tsim Sha Tsui area. Where can we buy the Airport Express Ticket? We will be going back to Phil from HK. Can we use the 3 days unlimited MTR ride first to travel HK? then our plan on the last day, riding the Airport Express train to HK airport? Is it possible?