Unity attribute to make Vector3 and Vector3[] fields draggable in the scene.

Draggable.cs

using UnityEngine;

/// <summary>
/// Attribute to add to Vector3 and Vector3[] fields to make them draggable in the scene.
/// </summary>
public class Draggable : PropertyAttribute
{
}

DraggableEditor.cs

using System;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

/// <summary>
/// Makes Vector3 and Vector3[] fields decorated with the Draggable attribute draggable in the scene.
/// </summary>
[CustomEditor(typeof(MonoBehaviour), true)]
public class DraggableEditor : Editor
{
    void OnSceneGUI()
    {
        if (Tools.current != Tool.Move)
        {
            return;
        }

        var transform = ((MonoBehaviour)target).transform;

        using (new Handles.DrawingScope(transform.localToWorldMatrix))
        {
            foreach (var property in GetPropertiesWithDraggableAttribute())
            {
                if (property.propertyType == SerializedPropertyType.Vector3)
                {
                    Vector3Handle(property, property.name);
                }
                else if (property.isArray && property.arrayElementType == nameof(Vector3))
                {
                    for (var i = 0; i < property.arraySize; i++)
                    {
                        var element = property.GetArrayElementAtIndex(i);
                        Vector3Handle(element, $"{property.name}[{i}]");
                    }
                }
            }
        }
    }

    IEnumerable<SerializedProperty> GetPropertiesWithDraggableAttribute()
    {
        var targetObjectType = serializedObject.targetObject.GetType();
        var property = serializedObject.GetIterator();

        while (property.Next(true))
        {
            var field = targetObjectType.GetField(property.name);

            if (field != null && Attribute.IsDefined(field, typeof(Draggable)))
            {
                yield return property.Copy();
            }
        }
    }

    void Vector3Handle(SerializedProperty property, string labelText)
    {
        Handles.Label(property.vector3Value, labelText);
        property.vector3Value = Handles.PositionHandle(property.vector3Value, Quaternion.identity);
        serializedObject.ApplyModifiedProperties();
    }
}