48" (length) x 24" (depth) x 18" (height) enclosures would probably house all three comfortably for life. If you end up with a female that reaches 8' I would recommend moving her up to 6' x 24" x 18". As BCR said, though. That isn't going to happen any time soon. Currently, my biggest Boa is only 4 1/2 feet long and he is still perfectly comfortable in a 41 qt. rack. Your smaller ones would be as well. So, you could always just buy/build one bigger enclosure at a time as needed if you don't want to spend all of that money at once on three of them. Your bigger female is definitely ready for a 4' enclosure right now, though...