Linear regression in 3D space?

I have a series of point in the 3D space. I want to find the equation of the regression line. I found a lot of material about how to find a and b if you have just x,y points (line equation: y=ax + b), but in this case I have to find a,b and c (ax + by + c) = 0 starting from a series of x,y,z points. Any suggestion?