They can be housed together, although many do not recommend this. I prefer keeping them seperated outside of breeding season because it will make it much easier to spot a problem with an invidual if one reguritates or something.
One thing I would recommend if kept together is that all be about the same size. Smaller boas may feel intimdated by larger ones in the same enclosure. Also, always feed seperate and watch them very closely when placed back together after feeding. Captive rosy boas have a very good feeding response and it's not uncommon for one to strike at and sometimes constrict another when they are worked up and excited about food.