This .NET class wraps the AcGeCurve3d ObjectARX class. This ia an abstract base class for all 3D curves. Any class that is derived from this class represents a 3D curve.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Curve3d
        Autodesk.AutoCAD.Geometry.CircularArc3d
        Autodesk.AutoCAD.Geometry.CompositeCurve3d
        Autodesk.AutoCAD.Geometry.EllipticalArc3d
        Autodesk.AutoCAD.Geometry.ExternalCurve3d
        Autodesk.AutoCAD.Geometry.LinearEntity3d
        Autodesk.AutoCAD.Geometry.OffsetCurve3d
        Autodesk.AutoCAD.Geometry.SplineEntity3d

Public abstract Class Curve3d
Inherits Entity3d

public abstract class Curve3d : Entity3d;