You must apply in all countries
Unfortunately, there is no such thing as an "international patent". Patent protection is effective only in the country (or territory) granting the patent. Even a PRC patent confers no protection for the invention in the Hong Kong SAR. So, if you sell your products in U.S., China and Japan, you have to apply for patents in all three countries.
Strategy to claim priority date
There are international treaties that enable inventors to defer applications for patents in different countries and yet claim the priority date of the first country. For example, under the Paris Convention (of which most industrial countries are members), you may make an application for your invention in the U.S. and then make another application in China within one year. The priority date of your China application could date back to the date of the U.S. application. Under the Patent Co-operation Treaty, the maximum deferral is 30 months. However, to benefit from the PCT, you have to make your application to the International Bureau of WIPO in Geneva, Switzerland, either directly or through a receiving office in your home country, and there are certain formalities to be complied, whereas the Paris Convention has no such requirements.
Why it is advisable to apply for patent in U.S. First?
In the above example, it is advisable to first apply for patent in the U.S. so that you have one year's time to consider whether you would like to patent your invention in other countries. The choice of the U.S. as the first country to apply is not arbitrary. As the U.S. has the most stringent and elaborate requirements for patent documentation, an application submitted to the USPTO would, when translated, suit the purpose for China and Japan, but the reverse may not be true. A Chinese application, when translated, may not suit for the U.S. purpose. And if the USPTO rejects the application for this reason, you may lose your right to obtain a patent in the U.S.
In case if you are concerned that the product life cycle of your invention may be very short, you can consider applying for a short-term patent. It usually takes at least 3 years to obtain a patent. The short-term, which is usually granted in around 6 months and has a protection period of 8 to 10 years, depending on the country of registration, is a convenient way to prevent others from copying your invention.
If your new product has market potential and you fail to patent it in the first opportunity, you will lose your right forever because of the novelty requirement explained above. It is not always easy to predict the life of your product's appeal. Very often the commercial life of a product may be prolonged by modification and improvement. However, if you do not patent it, and your competitor does, then you stand to lose not only the right to make and sell the original product, but even the modified or improved version of it.