1. Vitamin D3 is best at bedtime, since it makes you feel somewhat mellow. It is very good to sleep.
2. Melatonin works best in smaller dosage. Remember that the body produces only very tiny amounts of it. You should not expect it to make you sleep at first. It is a process, but within 2 or 3 months you can be sleeping like a baby.
Also, that amount of vitamin C is not too big. Just be careful not to take your afternoon dosage too close to the evening since it can interfere with your ability to sleep.