Hard to say how long you can keep the three dragons in that cage when you didn't mention how big each dragon is. I would separate them now if they are all different sizes, as the largest dragon can easily intimidate the smaller ones and eat most of the food, take the best basking spots etc. A single dragon in a cage that size should be move into their adult cage once they are about 15" long, head to tail. As you have three dragons in there, I suggest separating them into similar sized cages once they are 10" long head to tail tip. As smaller dragons don't always do as well in large cages to start, if you move them all into separate adult cages at the same time, partition off part of the cage so it's not so big at first and remove the partition once the dragon reaches 15" inches long.
If you are not 100% sure on the sex of each dragon, deffinitely have them all in separate cages by the time they are 6 months old (pretty much the time they will reach 14" or more in length, with proper care). Though two or three females can be housed together, they do not always get along and can turn on eachother at any point during their lives even if they had been getting along for several years. Males and females should not be housed together till both are at least 12-14 months old (male) and the female at least 15 months old, older much better. Two males should never be housed together. Rarely two brothers may get along fine in a large enclosure but this is more the acception than the rule.
A single adult bearded dragon should have at least 4'x2' of floor space and between 16" to 24" of height. For each additional adult dragon housed together, an additional foot should be added to the cage length or width. (ie 5'x2' of floor space or 4'x3' of floor space for two dragons, 6'x2, 4'x4' or 5'x3' of floor space for 3 dragons)
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PHLdyPayne