Since MeshGeometry3D is not a Control, ToolTip cannot be used directly. Instead, you can monitor MouseEnter and MouseLeave events and display ToolTip in them.
private void X_MouseEnter(object sender, MouseEventArgs e) { Point p = e.GetPosition(someObject); tooltip.Show("Это подсказка", someObject, p); } private void X_MouseLeave(object sender, MouseEventArgs e) { tooltip.Hide(someObject); }