works with three screens. Just have to change line 52

multiple-screens.ino

#include <Arduino.h>
#include <U8g2lib.h>

#ifdef U8X8_HAVE_HW_SPI
#include <SPI.h>
#endif
#ifdef U8X8_HAVE_HW_I2C
#include <Wire.h>
#endif

#define TCAADDR 0x70 // I2C Multiplexer ADDR

struct display {
  U8G2 *display;
  uint8_t i2cnum;
  const u8g2_cb_t *rotation;
};

//U8G2_SSD1306_128X64_NONAME_1_HW_I2C display128x64(U8G2_R0, /* reset=*/ U8X8_PIN_NONE, /* clock=*/ 4, /* data=*/ 0);   // ESP32 Thing, HW I2C with pin remapping
//U8G2_SSD1306_128X64_NONAME_1_SW_I2C display128x64(U8G2_R0, /* clock=*/ SCL, /* data=*/ SDA, /* reset=*/ U8X8_PIN_NONE);   // All Boards without Reset of the Display
U8G2_SSD1306_128X64_NONAME_1_HW_I2C display128x64(U8G2_R0, /* reset=*/ U8X8_PIN_NONE);

#define NUMSCREENS 3
display OLEDS[NUMSCREENS] = {
    { &display128x64, 1, U8G2_R3 },
    { &display128x64, 7, U8G2_R3 },
    { &display128x64, 2, U8G2_R3 }
};

/* I2C multiplexer controls */
void tcaselect(uint8_t i) {
  if (i > 7) return;
  Wire.beginTransmission(TCAADDR);
  Wire.write(1 << i);
  Wire.endTransmission(); 
}

void setup()   {
  Serial.begin(115200);
  while (!Serial);
  Wire.begin();

  tcaselect(1);
  OLEDS[0].display->begin();
  tcaselect(2);
  OLEDS[1].display->begin();
  tcaselect(6);
  OLEDS[1].display->begin();
}

void loop(void) {
  tcaselect(6);
  OLEDS[0].display->firstPage();
  do {
    OLEDS[0].display->setFont(u8g2_font_ncenB10_tr);
    OLEDS[0].display->drawStr(0,24,"Hello World!!!");
  } while ( OLEDS[0].display->nextPage() );
  delay(1000);
}