You can watch the Tamil dubbed version on streaming platforms like JioHotstar 300: Rise of an Empire (2014)
Keep in mind that there might be other Tamil-dubbed movies or regional productions based on the same story, but "300" (2006) is a well-known and widely available one.
"Innum dhooram... poagalaam," the narrator boomed in a deep, gravelly baritone that belonged more to a retired police officer in a Vijayakanth movie than an ancient Greek storyteller. ("We still have a long way to go...")
The phenomenon of the "300 Spartans" franchise in the Tamil film circle bridges the gap between Hollywood's visual mastery and Kollywood's love for heroic defiance. Key Movies in the "300" Franchise
After the merger of Viacom18 and JioCinema, Warner Bros. content often lands here. Recently, JioCinema has been aggressive in adding regional dubs. Check their library; you might find both 300 and Rise of an Empire with Tamil audio for free (with ads).
Highly stylized, high-contrast visuals, and slow-motion action. Focus: Pure adrenaline, machismo, and heroic sacrifice.