Trip to Bangkok, Hong Kong, Singapore
 

If I plan a trip for 12 days, can I do all three and give them justice? The 12 days would not include flying there and back.

No, you could not do them justice in that amount of time. You could get a sample of each. Hong Kong and Bangkok require the most time.

Meaning 4 full days in each city, plus 5 more days for flight days? Even with that, I'd say no. But, then again, what exactly are you hoping to do and see that would mean "give them justice" to you? Is there any reason you are considering going to 3 cities in 3 different countries in such a short period of time?

Asia is a great place to slow down. We always enjoy our travels more when we are able to really slow down and relax rather than race from place to place.

Absolutely not! Happy Travels!

drop singapore for this trip and you can get a taste of the other two...

Dissendent here. Yes, you can do all three in 12 days. 3 in Hong Kong, 2 in Singapore, about 3-4 in Bangkok. Rest for travelling.

It is highly doable and should be fun.

I agree with rkkwan, as you have said that the time does NOT include flights to and from the US, then you have 4 days in each city. Very good for a nice intro to each place. You could even cut a day in Singapore and add to Bangkok if you wanted. Flight time between the cities is quite short, and you will "gain" an hour between Hong Kong and Bangkok/S'pore. If you plan your inter-city flights for early morning or late night, and of course take non-stops, you will maximize your sight-seeing time in each city. The best itinerary for least backtracking and shortest flights would be Hong Kong-Bangkok-Sing.

Virtually all my friends and relatives who have come to visit me have had this amount of time or less and we have "done" these three cities (or one less and Bali) in your same amount of time. (How many Americans take more than 2 weeks vacation?!!)

It is doable as i have done it, however you will be rushing around trying to get everything in (i do it each holiday and then am full of regret as i never had time to just wonder and savour some of the other sights) Personally i love SP and feel it warrants more than 2 days.
So why dont you stick to either HK/SP and BK and then you can really explore.
BK is definately worth several days.