The is a phantom update for most users. Oppo abandoned the device at Nougat. While unofficial Android 8 exists through LineageOS, it is unstable and breaks core features like the dual-selfie camera that defined the F3.
Oppo does not provide an official bootloader unlock tool. You must use a paid service (like DC Unlocker) or find a test-point exploit (advanced users only). Oppo F3 Android 8 Update Download