About learning three languages--I don't think at 3.5 you have to worry one way or another. Language learning at that age is not a chore and if the child is enrolled in a decent school it's mostly about learning through play anyway.
My son is 3-years, 2 months-old and is in a local Cantonese Medium of Instructinon kindergarten. My husband is local Chinese so he speaks Cantonese and my son also speaks Cantonese with his grandparents. He speaks English with my husband and I.
He goes 1/2-days to kindergarten (about 3 hours/day) in which time he has lessons in Cantonese, English and Mandarin. I went to a parent-teacher conference this past weekend and the teacher said that my son is fluent in English and Cantonese and is messermized by Mandarin. The Mandarin teacher at their kindergarten is fabulous and makes the class really interesting.
I think that it's silly to think that children are being "forced" to learn Mandarin and that they won't use it. Maybe they won't immidiately use the language (i.e. at home) but Mandarin is a language of ever-growing importance worldwide. In the United States there are many primary schools that are starting to offer Mandarin courses (this was a recent CBS News story) and the waitlists for these schools are very long. Even in Hong Kong, I use Mandarin a lot (it tends to be easier for English-speakers to learn than Cantonese because there are fewer tones and the pronunciations are similar to English). As I don't speak Cantonese well, I often use Mandarin at restaurants and in taxis. I also like to travel to Mainland China and it is invaluable to be able to speak Mandarin acrosst he border.
Here in Hong Kong students are extremely interested in learning Mandarin. I studied Mandarin in university and teach primary school and my students love to speak with me in Mandarin (not so much in English, though).
So, you'll find that Cantonese is not even as practical world-wide as Mandarin will soon be. And what does it hurt to expose your child to many different languages--especially when at this age they literally can absorb languages like a sponge without any effort? Maybe later, even if you move away from Hong Kong, your child will remember his language learning experience and that can give him better confidence for picking up languages later on.
My take is let your child be exposed to as many new languages as possible because it won't hurt anything at all.