What to do if i took 3 caps of 400mg no flush niacin, am i gonna die?

Niacin: You are unlikely to die from that dose of niacin.

Agree, but: The no-flush Niacin has been shown to not have the same effects on triglycerides and lipid profiles. Don't use it. Take Niacin with a meal, and after a couple of weeks the flushing sensation usually minimizes.

Not from that.: Well, you won't die from taking 1200 my of Niacin at once. Truth be told, recommended doses are as high as 1500mg twice daily for improvement of cholesterol and higher-level lipid parameters. So the dose you took is actually quite normal. Thankfully you took the no-flush variety, because if you took that dose of regular Niacin you'd feel lousy with flushing and red face - again, not fatal.