Do not use D3 supplementation unless you know for a fact that your dragon has a D3 deficiency. The main purpose of UVB lighting, natural, fluorescent, or MVB, is so that your dragon creates it's own D3 on demand as needed. Overdosing on D3 can cause hypercalcemia which can be fatal or cause other serious internal injuries. I would only use a D3 supplement if told to do so by a licensed vet who has done blood work on your dragon and only then the specified dosage for the length of time told to. In most cases D3 poisoning can be far worse than a deficiency.
EDIT:
Here's an actual quote from a medical journal I have in the house, granted it's talking about people but the most of the same risks should apply to beardies as well....
"Vitamin D toxicity can cause non-specific symptoms such as anorexia, weight loss, polyuria, and heart arrhythmias. More seriously, it can also raise blood levels of calcium which leads to vascular and tissue calcification, with subsequent damage to the heart, blood vessels, and kidneys."