How should I extend this run?
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It's a fine grow-out coop, but you're going to need something with a bit more elbow room for long term. Overcrowding stress is no joke. It can cause all sorts of aggression problems, ranging from mild feather picking (which can develop into a life-long bad habit) to cannibalism.
That little coop would be fine for 3 bantams, but you've got 3 large breed chicks. Those girls are going to get a lot bigger. The average large breed hen will weigh about 5.5 to 6 pounds and has a wing-span of about 3.5 to 4 feet. They can take up a square foot, just standing still. They need room to run around, scratch, dust-bathe, and stretch their wings. All of that requires plenty of room.
While they are getting to the point where their growth rate is slowing, they will be out growing that coop by about 10 to 12 weeks old. By then, they will be about half-way to their adult size.
You will probably be able to build a simple coop for less than you spent on that prefab one.
There are lots of coop plans and ideas in the coop section. Most of them give really good instructions with measurements and materials lists. All you really have to do is choose one, print it out, and take it to you local hardware store. They'll help you purchase all the materials and even do all the major cuts for you. If you can use a drill, you can build a coop.