3 weeks is just about right. for most newcomers, you need about a week to find a suitable accommodation (unless you have a pre-arranged on-campus housing agreement with your DLI). you may or may not find a place to rent in just one week. unless you’d go for whatever is available. throw in another week to move in and settle down, and to buy furnitures if you have to. the week before classes start is usually reserved for orientations.