If it is three times per second, it's unlikely to induce seizures in those with photosensitive epilepsy. Though, depending on where you live, there may still be

If it is three times per second, it's unlikely to induce seizures in those with photosensitive epilepsy. Though, depending on where you live, there may still be rules against it (some places set the limit at 2Hz), so it may be worth reporting it. Flashing signs like that are still very rude, though.