**unterteilt** is a past participle from the verb "unterteilen", which is used with "in etwas (Akkusativ)". 
Many past participles* retain the original prepositions and cases of their respective verbs. An example, where a verb with something of a movement to it has a past(or passive) participle used with the same preposition and case would be sich verlieben:

> *Sich in jemanden (Akkusativ) verlieben - verliebt sein in jemanden (Akkusativ):*
> 
> Ich bin verliebt in sie, aber sie antwortet nicht.

Furthermore, you shouldn't think that always where there is a movement you should use Akkusativ, or, conversely, that if there is no movement, that you should always use Dativ. This is nothing but a heuristic rule of thumb that comes from the real language usage. You have probable seen it yourself at the example of the preposition "zu", that means movement but is exclusively used with Dativ. Language as a usage based statistical phenomenon doesn't care too much about any of these rules, which are nothing but approximations and/or mor or less successfull attempts to standardize the usage. 

On a side note, from the semantical standpoint there is no sense of locality in statements such as "Germany is divided **in three parts**", and in this regard it doesn't speak for the usage of Dativ. 
Here is a similar example:

> Ein Stück Marmor, zerbrochen in drei Splitter(Akkusativ, because not "Splitter**n**), lag auf dem Boden
> On the floor there was a marble block broken in 3 pieces.

Here we have the Verb "zerbrechen" being used the very same way as "unterteilen" in your example. 


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* one such example is the verb "sich für etwas (Akk) interessieren, which has the participle interessiert, which is used with the preposition "an + Dativ", i.e. "interessiert sein an etwas (Dativ)".