My understanding is that you can just use D3 but there are some risks and it is not as much of a sure thing your dragon is getting the right amount of D3 than with using a UVB bulb.
The UVB lets the reptile regulate it's own production of D3 instead of relying on you. It is possible to overdose on D3 and when that happens it can be very deadly.
I've also seen some experienced keepers post evidence of dragons not doing as well with just D3 as compared to using a UVB light. On the flip side I've seen some keepers argue it does not matter but I believe these keepers failed to post documented evidence.
Basically, I'd say if you're not very experienced with beardies don't risk the chance of giving the wrong amount by only using the D3 supplements. If you are experienced and feel like it, give it a shot and see how your dragon does. By experienced I mean able to recognize any signs of health problems or abnormal growth, etc.
If you go the supplement route it would be beneficial to record growth rates, etc. and eventually post them on this board. Eventually this may help clear up the debate.
Whatever you do I wish you both luck!
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Rob Talkington