D3 from supplements can build up in the chameleon's system and lead to health issues and symptoms of MBD. D3 produced from exposure to the UVB light (most often recommended UVB light...long linear fluorescent Repti-sun 5.0 tube light) should not cause an overdose as long as the chameleon can move in and out of the UVB. UVB should not pass through glass or plastic, BTW. Dusting twice a month with a phos.-free calcium/D3 powder ensures that the chameleon gets some D3 without overdoing it and leaving it to produce the rest from its exposure to the UVB.
D3, calcium, phosphorus and vitamin A are all important players in bone health and they need to be in a balance. They not only play parts in bone health but in other systems in the chameleon's body too. When balancing them, you need to look at what you feed the chameleon, what you gutload/feed the insects and what supplements you use.
There are two "kinds" of vitamin A involved...beta carotene (a.k.a. prOformed vitamin A) which is converted to vitamin A (by the body as needed in animals that are able to convert it....and its controversial for chameleons as to whether all/any of them can convert it). Beta carotene won't build up in the chameleon's system for sure though since it is only converted as needed. Then there is prEformed vitamin A (palmitate, acetate, retinol, etc.) that is exactly that...prEformed or already to go. This prEformed vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD....so it should be used with care.
Dusting twice a month with a vitamin powder with a beta carotene source of vitamin A will help to provide the vitamins the chameleon needs without overdosing the vitamin A....and leaving the chameleon's owner in charge of how much (if any) prEformed vitamin A to give the chameleon.
Since most insects have a poor ratio of calcium to phos. its important to dust the insects at most feedings with a phos.-free calcium powder to make up for it.
Appropriate temperatures allow the chameleon to better digest its food and thus indirectly in nutrient absorption.
Gutloading/feeding the crickets, roaches, locusts, superworms, etc. with a wide variety of greens (dandelions, collards, kale, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, sweet potato, sweet red pepper, zucchini, etc.) will keep the crickets healthy and thus help your chameleon too.
I know you only asked about D3 but all of this goes hand in hand. Hope this helps!