Quick Answer
Nippon Sangoku: The Three Nations of the Crimson Sun is streaming on Amazon Prime Video. 12 episodes (~5h total watch time). Availability may vary by region.

Where to Watch Nippon Sangoku: The Three Nations of the Crimson Sun
Nippon Sangoku: The Three Nations of the Crimson Sun is currently on Amazon Prime Video. Keep in mind that catalogs change by region — if you don't see it on your usual service, check the other platforms listed here.
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Anime licensing changes by region. If Nippon Sangoku: The Three Nations of the Crimson Sunisn't on your usual service, try these.
About Nippon Sangoku: The Three Nations of the Crimson Sun
The pinnacle of modern civilization known as Japan is now long forgotten. Ravaged by civil war and technological collapse, Japan has regressed into a post-Meiji era society and is now divided into three imperial territories known as Yamato, Buo, and Seii. However, Aoteru Misumi, a 15-year-old literature enthusiast, dreams to bring back the culture and technology that once made the pride of his country. Although unwilling to turn his theoretical knowledge into concrete action at first, a tragic...
Full details on MyAnimePulseFrequently Asked Questions
Where can I watch Nippon Sangoku: The Three Nations of the Crimson Sun legally?
Right now, Nippon Sangoku: The Three Nations of the Crimson Sun is on Amazon Prime Video. That said, anime licensing varies a lot by country — what's on Netflix in Japan might not be on Netflix in the US. If your go-to platform doesn't have it, try the other services listed above.
Is Nippon Sangoku: The Three Nations of the Crimson Sun on Netflix or Crunchyroll?
Can't confirm either right now. Both platforms rotate their catalogs pretty often, so it's worth searching directly on each one.
Is Nippon Sangoku: The Three Nations of the Crimson Sun available with an English dub?
Dub availability depends on the platform. Crunchyroll has been expanding their dubbed library a lot recently, and Funimation (now merged into Crunchyroll) historically had the biggest dub catalog. Check each platform's listing for Nippon Sangoku: The Three Nations of the Crimson Sun.
Can I watch Nippon Sangoku: The Three Nations of the Crimson Sun for free?
Crunchyroll offers a free ad-supported tier for some shows. Most other platforms require a subscription, but many offer free trials for new users. Tubi and Pluto TV occasionally have anime in their free rotation too.