Three adult hens can be enough if they are experienced and were used to a well behaved rooster.
I would give them some days of 'see but no touch' and then put him in the hens' territory.
If you put the hens in his territory he will have advantage over them which might encourage him to show...
His hormones are raging and the pullets are too young to deal with his needs and can get harmed.
You could separate him with 4 adult older hens to let them teach him some manners.