#include "FastLED.h"
#define NUM_LEDS1 10
#define NUM_LEDS2 10
#define NUM_LEDS3 10
CRGB strip1[NUM_LEDS1];
CRGB strip2[NUM_LEDS2];
CRGB strip3[NUM_LEDS3];

// 1 sec. frequency
unsigned long interval=1000;    // the time we need to wait
unsigned long previousMillis=0; // millis() returns an unsigned long.

// 5 sec. frequency
unsigned long interval1=5000;    // the time we need to wait
unsigned long previousMillis1=0; // millis() returns an unsigned long.

void setup() {
  FastLED.addLeds<NEOPIXEL, 5>(strip1, NUM_LEDS1);
  FastLED.addLeds<NEOPIXEL, 6>(strip2, NUM_LEDS2);
  FastLED.addLeds<NEOPIXEL, 7>(strip3, NUM_LEDS3);
}

void loop() {
for(int i = 0; i < NUM_LEDS1; i++)
for(int i = 0; i < NUM_LEDS2; i++)
for(int i = 0; i < NUM_LEDS3; i++)
 // other CMD's...
 strip1[0] = CRGB::Red;
 strip2[0] = CRGB::Green;
 strip3[0] = CRGB::Blue;
FastLED.show();
 strip1[0].fadeToBlackBy(255);
 strip2[0].fadeToBlackBy(255);
 strip3[0].fadeToBlackBy(255);
FastLED.show();
 // replace delay(1000)
 if ((unsigned long)(millis() - previousMillis) >= interval) {
    previousMillis = millis();
    // every first second
}
// other CMD's...
 strip1[1] = CRGB::Red;
 strip2[1] = CRGB::Green;
 strip3[1] = CRGB::Blue;
FastLED.show();
 strip1[1].fadeToBlackBy(255);
 strip2[1].fadeToBlackBy(255);
 strip3[1].fadeToBlackBy(255);
FastLED.show();

 // replace delay(5000)
 if ((unsigned long)(millis() - previousMillis1) >= interval1) {
    previousMillis1 = millis();
    // every fifth second
    // ...
}
// other CMD's...
strip1[2] = CRGB::Red;
strip2[2] = CRGB::Green;
strip3[2] = CRGB::Blue;
FastLED.show();
strip1[2].fadeToBlackBy(255);
strip2[2].fadeToBlackBy(255);
strip3[2].fadeToBlackBy(255);
FastLED.show();
}