Much depends on the rooster. Some roosters are so aggressive that they won`t leave a hen alone as she broods. Best to watch closely. Even if you don`t see him bothering her, she may show some tell tale signs, if he`s a bad boy. If he can`t stand being alone for 3 weeks, just feel his pain, or get him another girlfriend.
I had s a similar situation. I bought a house in FL that had a resident game hen. I felt sorry for her, so I bought a gamecock to keep her company. Along with the cock came another hen. Not long after, both hens started setting. The boy was distraught. They were all free range. The cock would roost as much as an hour before dark and not fly down until the sun had been up for a while. Any hour of the day it was possible to see him running through the bushes and around the yard for no good reason. After the eggs hatched, he was a great father and his sinces seemed to return.
Now, a young rooster and other hens, are a danger to chicks. A mature cock will usually be a good father. Be sure to observe and make sure the hen is up to the task of protecting her young, as some are not. Hope this little insight helps. Have fun.........Pop