How to fit 3D points in circle

Hello, I want to ask about fittinng point in 3D space. I have tubular structure where the external boundary coordinates [100 * 3] are available at certain points along the length of vessel. I want to approximate these available points with circle/ellipse for precise mathematical approximation of vessel boundary. The required is centre and radius of circle in 3D so that I can insert boundary points(circumference coordinates) in between two consecutive available borderlines.