You really don't want to get chicks at three weeks old. They're too old to be excessively cute (will have started feathering out), but still young enough for their care to be an (expensive) pain. At that age, they will need heat lamps and chick crumbs. At 6-8 weeks, they can be weaned off heat and off chick crumbs, onto growers pellets. From (not before!) 16 weeks or so, they can be weaned onto layers pellets.
Honestly, you'll be much better off buying chickens at somewhere between 16 weeks and point of lay (POL; 21 weeks). Not only will they be easier to care for, you'll actually have some hope of having eggs this summer (chicks born at this time of year may, or may not, dane to actually lay this side of next spring, as the shortening of daylight hours reduces egg laying capacity, sometimes to 0).
Introducing birds can be a royal PITA (and it's a spectacularly bad idea to put 3 week old chicks in with unfamiliar adults). They need to be in adjacent runs where they can see each other for a few days. The next bit has two schools of thought
a) put in the newbies at night, so that they all wake up together, hopefully imagining that it was always that way (they're not that bright...). However, you do need to be down at the coop at dawn to monitor things - I made that mistake once and found the newbie being attacked quite viciously.
b) chuck 'em in together in daylight, so you can see what happens.
This may, or may not, however, work first off. If it doesn't, you can either return the newbie to the adjacent run, or (if it's only one hen doing the attacking), move the perpetrator to the adjacent run, and leave the newbie in the main run.
If you don't have a spare run, then it is possible to fashion one v. cheaply using some stiff-but-flexible plastic trellis (I can't be sure, but this looks like the right stuff www.wilkinsonplus.com/netting+trellis/wilko-get-gardening-multi-mesh-50cmx5m/invt/0008666/?htxt=PsAGyAqy%2FDSGVBgOHPBfATKVETOKIWHcwqoICuDrG%2FxTcDPfxIrYzUvEu76RzzM6wutKTeo9AOCB%0AtDs76aYYKg%3D%3D ), tied together with some cable ties, and with offcuts of wooden batons around the bottom to weigh it down / keep the shape at the bottom (attach using hoop nails or whatever it is that they're called). Use tent pegs to secure it to the ground. Shelter / shade can be found in the form of an open umbrella over the top.
Obviously not a great idea if you have audacious daytime foxes (and they will need to be brought in overnight, and in particularly inclement weather), but ours has also served excellently as an isolation cage (with newspaper on the floor, in the utility room!) and a broody-breaker (with some of the same mesh as a floor, to make sitting uncomfortable, kept off the ground a few inches with some more scrap wood, to keep a breeze going around the breast . . . I diverge.