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Well, it all depends on the quality of life you want your chickens to have. Factories cram three chickens in a cage the size of a piece of notebook paper but I don't think anyone would call that enough room. I personally find even the 10 sqft measurements to be too tight for my tastes since the tighter the quarters, the harder it is to keep clean but to each their own. As for your coop, that gauge of wire doesn't look like hardware cloth in the picture. Be warned that the holes are big enough for a coop to reach in and pull a chicken through one piece at a time. As others have stated, you have far more predators than you may know simply because you've never had to look for them before now. Once you have some tasty chickens in there, you'll be surprised at what you will find trying to get in to them. And just because your dogs (which you said are small) can't dig in, it doesn't mean that a larger dog who may show up can't.
"Is it hard to put another chicken in their if one does die.?"
In your set up? Yes. You have no place for the four week quarantine needed to make sure that the other bird doesn't have a contagion before putting her with your survivor. I would suggest that you add some more length to your current run since that seems the best way to add some square footage for happier chickens. Then a third or fourth chicken wouldn't be as hard to keep for a more solid flock.