Three different countries mean 3 different operators, so three different SIM cards.
Do you have a smart phone? If yes you can just use Skype with wifi.

Thanks, JPDeM.

Yes. I'll have a smartphone, but given that I'll be out of the hotel pretty much 18 hours a day, I doubt I'll have wifi all the time. Unless, there are data cards available in each country?

3G SIMs are cheap and easily available in Hong Kong, for Hong Kong. China Mobile also have cards that you can buy in Hong Kong with both a HK number and Chinese number, though they're not 3G.

There are wifi (some free, mostly paid) everywhere in Hong Kong, but it's simply easier to use 3G since it's so inexpensive.

Not that easy to get a 3G SIM in China. No idea about Taiwan.

No difficult to get a 3G SIM in both China and Taiwan. You just have to buy one. Sold everywhere. There is a lot of free wifi in China but you canot obviously have it "all the time".

Thank you. What are the companies you recommend getting a 3G card for the three countries? Are they easily available? Can I pick one at the airport when I land?

You can buy a SIM card at the Taipei airport. You can buy a special package for tourists that should last the duration of your stay. I forget the details now but I had enough airtime and data for one week. In China, it depends where you land. sometimes you can buy it at the airport. Certainly possible in Beijing and Shanghai. Doesn't matter what supplier but if you have an iPhone then it is better to go with China Unicom if you wish to have 3G.

Thank you!

My experience with a Chinese SIM card was that once I left Shanghai, the city where I purchased the card from the largest telephone company, all calls I made in the rest of China were long distance, so I quickly ate up all my prepaid time without knowing it. I kept wondering why my husband wasn't reaching me, until he got word to me through the tour company. Call to him from hotel was surprisingly inexpensive, then he called me back for a more extensive conversation

Yes, when you get a SIM card you get a phone number of that city. When you move around you will incur long distance fees. But these are minimal and you can dial a code before the phone number to reduce the charge.

JPDeM: That's interesting to know. How would I know what the code is?