You must change a wheel on a C truck at the roadside. Which precaution is correct?

1. Jack the vehicle on a live lane with no warning devices
2. Loosen the wheel nuts fully before the jack takes any weight, standing in the path of traffic
3. Protect yourself and others first (position, warnings), secure the vehicle against roll, use the correct jacking point, and refit the wheel to the specified tightness
4. Use a piece of timber under the axle as the only support, then crawl under the truck