You can pick up a 3 SIM at the airport in HK. 3 has a shop right at the airport. No need to buy one from ebay. I did this last year when I went to HK.

As for Japan, you can do what I did and buy a data only sim from B-Mobile. They have a 30 day 1-GB data only SIM for a little over 3,000 yen. Way cheaper than the $22/day roaming from 3. The only caveat is, you need a Japan mobile number to activate the SIM (in other words, know someone that will activate the SIM from his/her mobile phone). A Japan land-line won't work. It has to be a Japan mobile number.

Here's the B-Mobile SIM I purchased for my recent trip to Japan: http://www.amazon.co.jp/gp/product/B005LAZDVK (this one is a micro-sim, useable in iPhone and iPad). They have a regular sim product also. The nice upside to this is they allowing tethering.