Import 3-level categories tree

I have to import a 3 levels categories tree. Like this:

parent_category sub-categories-level1 sub-categories-level2
WWWW001 WWW0001 WW00750, WW00752
WWW0010 ......., .......
WWWW002 ....... ........


Doing a csv like this:

parent_category categories
WWWW001 WWW0001
WWWW001 WWW0010
WWW0010 WW00750
WWW0010 WW00752

The categories tree results correcly made

I tried to simplify the import file in this way:

parent_category categories
WWWW001 WWW0001, WWW0010
WWW0010 WW00750, WW00752

It did the first line correctly (parent category WWW001 with 2 sub-categories: WWW0001 and WWW0010)
but create a new WWW0010 as top categories.

Also doing in in two steps (two file imported, one with the first line only, and the other with the second line only)
generated the same problem.

Is it possible only the first import file structure ?

Many thanks for your suggestion.
I imported successfully the 3-level catalog, following your example.
(I tested previously that structure, but, evidentely, I did some error)