Yes you can do x-ray. And start physiotherapy treatment

To start physiotherapy and to confirm healing take a xray done

With three weeks x-ray surgeon will find out the fracture healing stage. After how many days you are planning to remove the slab? Based on that you can take an x-ray to know the further healing.

You can get an x-ray done to just confirm the healing

X-ray would give you a confirmation about the RE unification of the fracture fragment so that you can progress to Physiotherapy easily.

As long as the slab is there, physio will be limited to joints and muscles away from the fracture site, and given you are young the healing time is expected to be around 4-5 weeks for your upper extremity fractures , so PT is safe to go for... If there is any unexpected delay in recovery an X ray is required to rule out any healing issues.