Controlling 2 WS2812b strips with a NANO and 3 buttons

Other Hardware LEDs and Multiplexing
9

Why are you using classes for something that is so obviously not suited to classes?

You are making the code ten times more complicated than it needs to be.
Also

PushButton3 pushbutton3 = {9};

Wrong sort of brackets.

This code is just silly

void press2(){
        leds[1] = CRGB::Blue; 
        FastLED.show(); 
        delay(30);
        leds[0] = CRGB::Blue; 
        FastLED.show(); 
        delay(30);
}

What is wrong with :-

void press2(){
        leds[1] = CRGB::Blue; 
        leds[0] = CRGB::Blue; 
        FastLED.show(); 
}