Just get one chicken. You coop is small, and it will be very tight for 3 birds, two of them strangers to the home girl.
One on one, there will be a dust up or two, but it will quickly settle. I would add some clutter to your run, you are wasting all that vertical space for the birds. And a hide out where a bird can get out of sight would be helpful, a large piece of cardboard would work, a ladder, or a saw horse.
Another trick that does not need building, is to let the home girl out, and lock the new birds in the coop/run. This lets them calmly explore the new area, it lets the home girl see them in the home area. Feed along the fence.
If you are nervous, put the new girls in the dog crate at night, and then let the home girl in to roost. Do that two days.
Then let everyone out together, with the choice to go in the run or yard. I am betting they will just all go to roost that night.
The quarantine is hard to do, and if you don't do it right, you may as well not do it. If you go into a decline if you lose a bird, then don't get strange birds, raise up chicks. Quarantine if incredibly important if you have a very large flock, derive your income from the birds, or have a flock of rare genetics. In those cases, I would have a closed flock, and not add birds, there is a risk.
But if you are just risking a few birds of hatchery birds, then the advantage of getting them is greater than the risk. Do not take anything you are sorry for, do not take anything that has parasites. If the people go to a lot of shows, or bird auctions, I would pass. But if they are just have a chicken hobby like me, I would risk it.
Mrs K