public GameObject a;
public GameObject b;
public GameObject c;
int counter = 0;
int max_count_object = 3;

public bool select = false;

void Update()
{
    selectedTest();
}

public void selectedTest()
{
    if (Input.GetKey(KeyCode.K))
    {
        switch (counter)
        {
            case 0:
                a.SetActive(true);
                b.SetActive(false);
                c.SetActive(false);
                Debug.Log("1");
            case 1:
                a.SetActive(false);
                b.SetActive(true);
                c.SetActive(false);
                Debug.Log("2");
            case 3:
                a.SetActive(false);
                b.SetActive(false);
                c.SetActive(true);
                Debug.Log("3");
        }

        counter++;
        if (counter > max_count_object) { counter = 0; }
    }
}

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

public class checkobj : MonoBehaviour
{
    public GameObject a;
    public GameObject b;
    public GameObject c;
    public int counter = 0;
    int max_count_object = 2;

    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        selectedTest();
    }

    public void selectedTest()
    {
        if (Input.GetKeyUp(KeyCode.K))
        {

            switch (counter)
            {
                case 0:
                    a.SetActive(true);
                    b.SetActive(false);
                    c.SetActive(false);
                    Debug.Log("0");
                    break;
                case 1:
                    a.SetActive(false);
                    b.SetActive(true);
                    c.SetActive(false);
                    Debug.Log("1");
                    break;
                case 2:
                    a.SetActive(false);
                    b.SetActive(false);
                    c.SetActive(true);
                    Debug.Log("2");
                    break;
            }

            counter++;
            if (counter > max_count_object) { counter = 0; }
        }
    }
}