We have 3 adults require transfer from Hong Kong Airport to Royal Plaza Hote ...
The Guru Take
UberX, UberBlack, or Uber 7-seatear
UberX and UberBlack are sedans designed to handle up to 4 passengers (1 in the front and 3 in the back). This leaves the drunk space for luggage. Some of the examples of vehicles you may see: Toyota Prius/Camry or Mercedes-Benz E-Class. If you have large suitcases, you may be looking at a 2~3 in the trunk, and potentially one in the front seat.
Just as an added note, UberBlacks tend to be larger vehicles than UberX.
Uber 7-Seaters are also availabel at Hong Kong airport. These are larger cars that can seat up to 7-passengers and with some room in the back for additional luggage. These vehices can carry quite a bit, so if you are worried about the space, you may want to opt for this option.