I recommend 6 separate cages, each being 4'x2'x2' for the adult cage. For now you can house them all separately in 30-35 gal tanks, which is roughly 36"x16"x20". The reason for this is you have females who are under 4 months and since dragons can't be accurately sexed till at least 6 months of age so one or more may turn out to be males. Also, they may not all get along throughout all their lives. They may be fine now (the 3 females you currently have) so it's better to get then all separate cages. Also, since all have alot of growing to do, some may grow faster than others and thus, intimidate the smaller and get all the prime basking areas, and food.
The males should be housed separated at the very lastest, 5 months of age. They come into sexual maturity alot sooner the females and can fight with eachother. Some males have been housed together from birth and lived through adulthood without problems but I believe this is the exception more than the norm. Many dragons have seriously injuried eachother in fights both as adults and accidently as hatchlings (nipped toes, limbs and tails).
Since females should be at least 15 months old (most breeders recommend 18 months or more) before breeding you have a long time to wait before breeding, a year or more. If your females do wind up all getting along and can be housed together their cage should be at least 6'x2'x2'. The male should only be introduced for a couple hours every day under supervision till the deed is done a couple times, then returned to his own separate enclosure. This offers the best protection to your females and your male (females can attack and injure a male if they decide they don't like him).
-----
PHLdyPayne