These are for the HSK test levels 1 & 2. Those are the lowest two levels. I bought one text for level 1 and one text for level 2. The third book is a collection of 5 previous tests of level 2. Both of the the textbooks have a mock test at the end of the book.
In case you are unable to tell, the books are written in Japanese. Each book has a CD as well.
The level 1 test requires knowledge of only 150 words. The level 2 test adds another 150 words for a grand total of 300 words!