Louis III was a French noble.
Most of the third generation Koreans residing in Japan cannot speak Korean.
I watch the anime "Lupin the Third" every day.
The three generation is the third generation. You can use this alone to say you're third generation Japanese or you can specify and say you're third generation something else.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.