Its it okay for me to take multivitamins, vitamin d3, and 300 mg of calcium all at once before i go to sleep?

Yes&NoSeveral Issues: Except for vit. D3 (commonly ↓ in US: unhealthy low fat foods), there is no clear evidence that multivitamins offer any health benefit: http://goo.gl/hTgfHM. If vitamin D3 concentrations are healthy, e.g. 70-80 ng/ml range, then high odds you will absorb all the Ca+2 you need from foods & tablets not helpful. The digestive system handles trillions different molecules in food ingested together.

Sure.: Taking all of those together at night shouldn't pose a problem. Some people do get stomach aches when taking vitamins without food, not otherwise it should be Ok.