Is there Point3D?

System.Windows.Forms.DataVisualization.Charting has Point3D class.

Represents the coordinates of a three-dimensional (3D) data point. This class is used when performing custom drawing with 3D charts.

• X     Gets or sets the X coordinate of a 3D point.
• Y     Gets or sets the Y coordinate of a 3D point.
• Z     Gets or sets the Z coordinate of a 3D point.

Also has Point3D structure.

Represents an x-, y-, and z-coordinate point in 3-D space.

• DirectX has a Microsoft.DirectX.Vector3 Structure, but will it be overkill for your application?
• XNA has class Microsoft.XNA.Framework.Vector3
• Unity3D has a Vector3 class for Representation of 3D vectors and points.
• OpenTK also represents a 3D vector using three single-precision floating-point numbers.

System.Windows.Forms.DataVisualization.Charting has a class Point3D

• float X, Y, Z
• System.Windows.Forms.DataVisualization.dll (WinForms)
• .NET Framework >= 4.0

System.Windows.Media.Media3D has a struct Point3D

• double X, Y, Z
• PresentationCore.dll (WPF)
• .NET Framework >= 3.0
• .NET Core >= 3.0

I know that Vector3D is not a Point3D, but if you just want a struct with X, Y, Z:

System.Windows.Media.Media3D has a struct Vector3D

• double X, Y, Z
• PresentationCore.dll (WPF)
• .NET Framework >= 3.0
• .NET Core >= 3.0

System.Numerics has a struct Vector3

• float X, Y, Z
• System.Numerics.dll
• .NET Framework >= 4.6
• .NET Core >= 1.0

Only System.Numerics.Vector3 does NOT depend on WinForms or WPF!