@Zhenyu Han
Interesting situation. Are you renting them on a short term basis (since you mentioned Airbnb)? Are you renting out each room or the whole unit?
There are several components to dissecting this. Short or long term, by the room or by the unit, and Craigslist or Airbnb/VRBO/Homeaway.
I think these components will naturally fit together as such (based on being in Honolulu):
Short term, by the unit, and Airbnb/VRBO/Homeaway - I believe short term rentals are best rented by the unit (not by the room) and are best marketed on VR-type websites.
Long term, by the room (or unit), and Craigslist - long term rentals could be by the unit or room and are best marketed on Craigslist.
In Hawaii, Craigslist is pretty comprehensive when it comes to long term rentals. It should generate you enough leads to find good tenants.
If there is an issue with not finding enough tenants, it might be due to some of the above key components not lining up effectively (what you're offering is conflicting with the website you're using).
I am intrigued by your situation. If I am understanding correctly (there's a good chance I might not be); you are renting three units and are now wanting to rent out each room in each unit. Fascinating! I have never heard of someone doing it on that level (multiple units). I always enjoy a new puzzle to think about. This is definitely one.