Need advice on jersey giants
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A real big rooster like that and only 3 hens - I think you may very well have barebacked girls. Of course you could always get another dozen hens or so. Only trying to help here.
There is always posturing and scuffling when changing the social structure of a flock - chicken society can be cruel. If it were me, I would get a third chick, raise the three together, and when large enough introduce them to the Australorp hen. There will be some fighting, but the older hen will spread her 'displeasure' among the three younger birds. "Given enough space they will work things out.
I have never raised Jersey Giants, but theoldchick has a flock of them. Drop her a private message with any questions. She's a really good person and will willingly help you.