Please look at DGR Table 1.5.A. You will notice that Category 3 is a qualification for Shippers and Packers and do not cover all the items. If you look at the requirements for Category 6, you will notice that it covers everything. With a Category 3 qualification, you are not eligible to attend a Category 6 recurrent course for extending your validity.
You will need to attend a Category 6 initial course and start anew. With a Category 6 course, please take note that there are two types of courses, the first type including Radioactive Material and the other excluding Radioactive Material. You will need to check before you subscribe to the course. Kinoshita Aviation Consultants is giving “Category 6 including Radioactive Material” courses which covers all the items including radioactive material.
As far as I know, there is no class in Japan conducting Category 3 courses. Category 3 qualification is not internationally common. Acquiring a Category 6 with Radioactive Material course qualification, you will have no difficulty in renewing whether in or out of Japan.