handler, I want to run periodically

Try the below

m_Handler = new Handler();
mRunnable = new Runnable(){
    @Override
    public void run() {
        if(count == 0){
            // do something
            count = 1;
        }
        else if (count==1){
            // do something
            count = 0;
        }
        m_Handler.postDelayed(mRunnable, 3000);// move this inside the run method
    } 
};
mRunnable.run(); // missing

Also check this

Android Thread for a timer

You should go for Timer and TimerTask in that case. Below is a small example:

//Declare the timer
Timer t = new Timer();
//Set the schedule function and rate
t.scheduleAtFixedRate(new TimerTask() {

    @Override
    public void run() {
        //Called each time when 1000 milliseconds (1 second) (the period parameter)
        //put your code here
    }

},
//Set how long before to start calling the TimerTask (in milliseconds)
0,
//Set the amount of time between each execution (in milliseconds)
3000);

Hope this is what you needed.