Can I take 3 mg of melatonin if I took 15mg of clorazepate 6 hrs ago?

You don't say your age or why you are taking both these drugs. I assume that you have some anxiety and insomnia, perhaps?
Keep in mind that the body produces melatonin. A lot of the modern world's habit of spending every moment indoors under lightbulbs of various kinds, not natural light and darkness cycles, has an effect on it's production, and you can do something about that.
Melatonin seems to have other beneficial effects on the body besides sleep, but so far, I don't see studies past the 3 month mark, so for now, it is suggested that you don't take it regularly past 3 months.
I can find no literature on any contraindications for your two drugs.

I am someone very aware of dawn, dusk, and every moment of sunlight in between, and find that it helps me a lot to be awake as the sky lightens, not just in as much natural light as I can. I have no idea why dawn helps but it does.