の is a very flexible particle, but there is one strict rule: the last noun is the main noun. Others serve as modifiers.
- 日本人の医者 a Japanese doctor / a doctor who is Japanese
- 医者の日本人 a Japanese person who is a doctor
If you want to say "a 30-year-old female Japanese doctor", doctor is the main word, so your translation should not end with 女 or 日本人. Besides this rule, the order of modifies is flexible as long as changing the order does not introduce ambiguity. In your case, the following phrases are all grammatical and at least understandable.
- 30歳の女の日本人の医者
- 女の30歳の日本人の医者
- 30歳の日本人の女の医者
- 日本人の30歳の女の医者
- 女の日本人の30歳の医者
- 日本人の女の30歳の医者
But native speakers don't usually say something like this in reality. Using three or more の's in succession tends to be seen as unsophisticated. You can reduce at least one の by using 女医 ("female doctor") instead of 女の医者:
- 30歳の日本人の女医
In this case の after 日本人 can be omitted, too, because ～人 is often used like a prefix:
- 30歳の日本人女医
Lastly, please keep in mind that changing the word order prematurely may result in a change in meaning.
- 30歳の女の話 a story of a 30-year-old woman
- 女の30歳の話 (incorrect)
- 太郎の友達の本 a book of Taro's friend
- 友達の太郎の本 a book of my friend Taro