call Tick event when timer starts

You can always call your method manually:

private void InitializeTimer()
{
    counter = 0;
    t.Interval = 750;
    t.Enabled = true;
    timer1_Tick(null, null);

    t.Tick += new EventHandler(timer1_Tick);
}

Just create a method then call this from within your timer and also just before you start your timer.

private int counter; 
Timer t = new Timer(); 

private void InitializeTimer() 
{ 
    counter = 0; 
    t.Interval = 750; 

    DoMything();
    t.Tick += new EventHandler(timer1_Tick); 
    t.Enabled = true; 
} 

private void timer1_Tick(object sender, EventArgs e) 
{ 
    if (counter >= 3) 
    { 
        t.Enabled = false;                 
    } 
    else 
    { 
        //do something here 
        counter++; 
        DoMything();
    } 
} 


private void DoMything()
{
   //Do you stuff here
}

You could use a System.Threading.Timer.

This has a constructor that takes an initial wait period. Set this to zero and the timer will trigger the callback immediately then every interval you specify thereafter.

Timer stateTimer = new Timer(tcb, autoEvent, 0, 750);

The simplest way might be to initially set the Interval to something very small, then increase it in the Tick handler, or in a separate handler.

This will ensure the first "tick" occurs in the same manner as subsequent ticks, e.g. on its own thread, rather than in the context of the initialize method.