I realised....

using UnityEngine;
using System.Collections;

public class characterSelection : MonoBehaviour {

    public bool firstIsEnabled = true;
    public bool secondIsEnabled = false;
    public bool thirdIsEnabled = false;
    public Transform firstCharacter;
    public Transform secondCharacter;
    public Transform thirdCharacter;

    void Update () {
        if (Input.GetKeyDown(KeyCode.F1) && firstIsEnabled == true) {
            GetComponent<characterController>().enabled = false;
            secondCharacter.GetComponent<characterController>().enabled = false;
            thirdCharacter.GetComponent<characterController>().enabled = false;
        }

        if (Input.GetKeyDown(KeyCode.F1) && firstIsEnabled == false) {
            GetComponent<characterController>().enabled = true;
            secondCharacter.GetComponent<characterController>().enabled = false;
            thirdCharacter.GetComponent<characterController>().enabled = false;
        }

        if (Input.GetKeyDown(KeyCode.F2) && secondIsEnabled == true) {
            GetComponent<characterController>().enabled = false;
            secondCharacter.GetComponent<characterController>().enabled = false;
            thirdCharacter.GetComponent<characterController>().enabled = false;
        }

        if (Input.GetKeyDown(KeyCode.F2) && secondIsEnabled == false) {
            GetComponent<characterController>().enabled = false;
            secondCharacter.GetComponent<characterController>().enabled = true;
            thirdCharacter.GetComponent<characterController>().enabled = false;
        }

        if (Input.GetKeyDown(KeyCode.F3) && thirdIsEnabled == true) {
            GetComponent<characterController>().enabled = false;
            secondCharacter.GetComponent<characterController>().enabled = false;
            thirdCharacter.GetComponent<characterController>().enabled = false;
        }

        if (Input.GetKeyDown(KeyCode.F3) && thirdIsEnabled == false) {
            GetComponent<characterController>().enabled = false;
            secondCharacter.GetComponent<characterController>().enabled = false;
            thirdCharacter.GetComponent<characterController>().enabled = true;
        }

        if (GetComponent<characterController>().enabled == true){
            firstIsEnabled = true;
        }

        if (GetComponent<characterController>().enabled == false){
            firstIsEnabled = false;
        }

        if (secondCharacter.GetComponent<characterController>().enabled == true){
            secondIsEnabled = true;
        }

        if (secondCharacter.GetComponent<characterController>().enabled == false){
            secondIsEnabled = false;
        }

        if (thirdCharacter.GetComponent<characterController>().enabled == true){
            thirdIsEnabled = true;
        }

        if (thirdCharacter.GetComponent<characterController>().enabled == false){
            thirdIsEnabled = false;
        }
    }
}