What will be the best option for transportation roundtrip for 3 persons

U can take airport shuttle bus for the round trip travel. At HKIA, take bus no. E41 and get off at PO HEUNG BRIDGE TAI PO. The Sun Hing Building is only around 250 yd away. The single trip takes around 70min and the bus fare is 13.9HKD. BTW, for the back trip, walk to KWONG FUK ROAD TAI PO to take the bus to HKIA.

Afterwards, I got some answers. Isn't it that Kowloon Mongkok Guest Hosue is in Mongkok, Kowloon so the citybus should be A21 and stop at #8 in Nathan Rd?

Sorry Gina. My mistake. Yes, you can take bus no. A21 and get off at Sino Centre, Nathan Road. Then you can find your guest house across the street. It takes around 55min and costs 33HKD.