The reason one of your dragons is not growing is your setup and feeding is insufficient for any of the three dragons. As you didn't give any sizes and weights on the dragons there is no way to know even if your larger ones are eating enough.
What I suggest, is separate all three dragons. The two larger ones should be housed in cages at least 4'x2' of floor space. The little could be alright in the 30 gallon til she starts to grow again (or till she is about 14" head to tail tip).
Make sure all setups have appropriate basking spot and UVB lights, keeping in mind ideal temperatures for the warm and hot end. Basking spot should have ranges between 95F to 120F with the hotter temp being the top or highest accessible part of the basking spot. The air temperature should be upper 80's lower 90's F and the cool side air temp should be around 75F and fall to room temperature at night (no lower than 65F).
Substrate for now can be just plain paper towel.
Get yourself a decent thermometer with probe and temp gun to measure temperatures accurately. Also buy a gram scale that can weight in 1 gram increments. You can find these at most kitchen supplies stores or department stores. Weigh and measure your dragons and keep track of this.
Next, get rid of the bugco stuff or use it to gutload your crickets. Pellet foods really should only be a topping to a wholesome salad for bearded dragons, not a staple. As at least one of your dragons is underweight and sized, I would start offering crickets of the appropriate sized (length of space between dragon's eyes or less) that are properly gutloaded (24 hours before feeding to your dragons) twice a day, all she can eat in a 10 min period or till she no longer shows interest. Have greens available all day.
If crickets are hard to find, or very expensive, try mail order in bulk instead of just buying a few at a pet store. Most places will sell 1000 crickets for $20 or less, some may even include shipping.
Also make sure your dragon is well hydrated as well. Weigh your dragons regularly, once or twice a week is good to monitor any weight gain or loss.
For a guideline on what are the best greens to use, check the nutrition chart here:
www.beautifuldragons.com
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PHLdyPayne