You cannot possibly know if a technical employee (at this level) is incompetent after just 3 weeks. Not possible.
It's the same as firing a CEO 3 weeks into the job... It doesn't make any sense unless that CEO did something blatantly illegal/dodgy during those 3 weeks.
The reality is you won't know how good they are until at least 6 months in.
As an engineer, 3 months is the absolute minimum amount of time I need to figure out if I'm a good match for a company.