Text A is generally for the place you are belonging to. ex. Dojo name, country or city, and so on.
Text B is usually for your surname in Japanese. The most popular and well understanding way is for this name to be written in Katakana - You may prefer it in Kanji so this is also available!
Text C is usually for your surname in alphabet, so that non-Japanese readers can recognise your name.
Any options can be left blank by simply entering "BLANK" in the corresponding field.