You can watch the movie on your mobile device through several legal streaming services, though free options vary depending on your location and existing subscriptions. Free Options (Ad-Supported)
If you already pay for these services, you can stream 300 at no extra cost: Watch 300 | Netflix 300 Movie Free For Mobile
If you want to watch without a paid subscription, these platforms often host the film for free with commercial breaks: You can watch the movie on your mobile
: Often has 300 available for free streaming with ads on its mobile app. 300 Movie Free For Mobile