We generally take the sows to the boar,as FF says he is then on home ground and concentrating on the visitors !
The sows can stay with the boar until a week before they farrow,so you could run all three together and only remove just before farrowing.
You may want to also think about timing, if you put both girls in with him then it is likely that you will have litters within 21 days of each other,then large period of nothing.