As you have read, HKG is very strict. It is not really CX. It is the airport authority. Unless specifically tagged by the operating airline, a passenger is entitled to one carry-on and one small piece (notebook or suitcase). The carry-on is limited to a certain size and should be 7kg or less (the notebook/suitcase is extra but I believe should be 4kg or less, and must be SMALL).

CX did weigh my carry-on before even though my carry-on was within the size limit, but I believe they just liked to make sure that I would not be stopped when entering the departure hall.

Your problem will be that you have three - a backpack, a notebook and a briefcase. You need CX to allow it, and they may not unless you are in a premium cabin or if you are a MPC SL/GO/DM (or perhaps OW SA/EM). Note that except for the documented benefits, all others are subject to their discretion. You may consider packing your notebook/carry bag in your briefcase or you backpack.