When you think that most countries have fought wars, this would be relatively easy by comparison. I agree it would be hard but not impossible. it isn't comparable. Essentially if people go off to war, the world continues with fewer resources.
You'd need nurses and doctors available to cover for people who have accidents, ambulances to drive them, cleaners to clean the hospital, petrol stations to fuel the ambulances, catering staff to feed patients. Pharmacists to dispense medication.
Hospitals will already have patients in, they need to continue to be cared for so add dieticians, physios etc too.
Midwives and assoc staff so also NICU etc staff.
There needs to be provisions for power failures or issues in the hospital, maintenance issues etc.
The fire service would also need to be fully operational and the police on at least a reduced staff.
People will still die so all the staff linked to that (you can't just stockpile bodies for three weeks).
Power plants etc so we have gas and electricity.
Yes you could survive three weeks on not-fresh food, but who's going to make sure people who can't afford to buy that in one go have enough food? Is the govt going to supply every house with a specific volume of food per person? That they have the skills to cook food that doesn't reply on sandwiches, toast, fresh food etc. Even in a basic issue, three weeks worth of nappies for three kids and three weeks worth of formula for two fat kids costs a fortune.
And how are they getting to work? Not everyone drives, not everyone can walk, so we still need buses and taxis and petrol stations.
So you could close entertainment, education and shops. Not much else.