As a mother of twins (and an older child), I've seen my share of single and double strollers. I had a Mountain Buggy Urban Double with board for my oldest which was great in the US (where they were born) but I couldn't get rid of it fast enough when we got here. I also brought my MacLaren double which was fine (not great) but it fits through doors, folds easily, good for travelling, etc. but again, HK is not great for double (side by side) strollers. We soon started using 2 singles for travelling and going somewhere and we just had them walk from early on. My oldest was able to walk everywhere from age 3 without a problem (and I used the board in the US for standing). Since you already have a 3yo I am seriously wondering why you would even get a large, bulky double stroller ... Have you thought about a buggy/kiddy board behind a stroller. When my twins were still have little and my oldest would get tired (while travelling, so we only had the MacLaren), I would put the lightest twin in a hip carrier, and have her sit down. So considering their age, I think you may have a few other options, without having to invest in a double stroller. If you really want a double stroller, I would advice Phil & Teds at this age. Your youngest could probably ride the bottom until 18 month (I think), by that time your oldest will definitely be fine walking. My personal experience (partly of having no choice and refusing triple strollers) is the more chances you give them to sit down, the more they will. The more they walk, the sooner they'll see it as their only option and they'll quickly get better at it. Sure, you have to make a stop here and there, but with kids you need to anyway. Hong Kong is not side by side double stroller friendly is MY experience, regardless whether it fits through doors (because almost all do).
Hope this helps?!