> this is the only company that I’ve worked for where you need a 3 week notice to request a day off.

That's Starbucks policy. Schedules are written three weeks in advance.

The reason for finding someone with the same skill set is this: the schedules are made with skill sets in mind along with availability etc. Is it necessary? Should it be THE deciding factor? All I can say is if you try your best to find someone of similar or close to skill set to cover then it should be doable. In my store most of the time we are just grateful that someone has covered instead of being short staffed.