How much of vitamins can i take a day?

Fish oil and flaxseed oil are not vitamins. You can take pretty much what you want of those, but too much of the fish oil will give you indigestion and possibly diarrhea. But not toxic in any way. Also, vitamin C is non toxic. Again, if you reach something called "bowel tolerance" on vitamin C you can get a diarrhea. So within reason, those three things you have listed are all safe to take a couple of a day with no trouble.