Change it after, especially if you are having the oil filter changed at the same time (which is often recommended.)
It is always possible that the mechanic will carry out the job incorrectly, and fail to replace the plug or filter properly. The filter gasket is quite large. The chances of a problem are low but I wouldn't leave a vehicle (or a house) alone after any event that could potentially cause a leak.
If you leave a car with an oil leak for 3 weeks you will have a garage full of oil, and a car that is undriveable. If you notice a leak between getting the change done and leaving on your trip, you will have to rush back to the garage to get it fixed.
If you do it after you come back, you should notice a leak immediately and be able to take corrective action without a sense of urgency. You will also find it easier to get a fix from the garage if you bring the faulty work back immediately rather than waiting 3 weeks.