Question6.cs

using UnityEngine;

public class Question6 : MonoBehaviour {

    public Vector3[] positions = new Vector3[]
    {
        new Vector3(0,0,0),
        new Vector3(-5,0,0),
        new Vector3(0,5,0),
        new Vector3(0,0,5),
        new Vector3(5,-5,5),
    };
}

Question6Editor.cs

using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(Question6))]
public class Question6Editor : Editor
{
    void OnSceneGUI()
    {
        Question6 question6 = (Question6)target;

        for (int i = 0; i < question6.positions.Length; i++)
        {
            question6.positions[i] = Handles.DoPositionHandle(question6.positions[i], Quaternion.identity);
        }
    }
}