3 LED blink

I will hopefully not get "arrested" with this question so here I go :o

I have made a small sketch with two blinking LEDs on pin 3 and 4 in the loop:
int redLED = 3;
int greenLED = 4;

however; I would like a third LED blink, but only after XX seconds = 20000ms
Could someone give me a hint or a code?

int redLED = 3;
int greenLED = 4;
int blankLED = 5;
void loop() 
{
digitalWrite(redLED, HIGH);     // turn the LED on (HIGH is the voltage level)
digitalWrite(greenLED, HIGH);   // turn the LED on (HIGH is the voltage level)

// here I want the blankLED to go HIGH in sync with redLED and greenLED but only after for instance 20 seconds
// then I want blankLED to go LOW in sync with the two other LEDs and blink in sync after that

delay(800);                     // wait for 800 ms = 0,8 second

digitalWrite(redLED, LOW);      // turn the LED off by making the voltage LOW
digitalWrite(greenLED, LOW);    // turn the LED off by making the voltage LOW
delay(800);                     // wait for 800 ms = 0,8 second

}

See blink_without_delay example and the countless threads and guides on doing more than one thing at a time / using millis() for timing. A great deal has been written about this, as it's a question that everyone learning Arduino has shortly after they make their first project.

// Defining LEDs
int redLED = 3;
int greenLED = 4;
int blankLED = 5;
int del = 400; // Milliseconds of delayment
void setup() {

// Making the LEDs outputs
pinMode(redLED,OUTPUT);
pinMode(greenLED, OUTPUT);
pinMode(blankLED, OUTPUT);
}

void loop() {
// blink blonk
Serial.print("Blink Blonk");
// Making every LED blink.
digitalWrite(redLED,HIGH);
delay(del);
digitalWrite(redLED,LOW);
delay(del);
digitalWrite(greenLED,HIGH);
delay(del);
digitalWrite(greenLED,LOW);
digitalWrite(blankLED,HIGH);
delay(del);
digitalWrite(blankLED,LOW);

}

Well, that was a waste of a post, wasn't it?