void setup()
{
  Serial.begin(9600);
 }
void loop()
{
  int a= analogRead(3);
  Serial.println(a);
  delay(1000);
}

but i have not connected anything to that analog pin but i am getting values around 500.whats the reason for that

 

Hard to say exactly why it could be say noise pickup. But a thumb rule in electronics is. Undefined input will get you undefined output. So measuring some floating input (not connected) do not give any meaning at all. It is like a radio tuned between to stations