Cpp
#include <SPI.h>

constexpr int TFT_SCK   = 18;
constexpr int TFT_MOSI  = 23;
constexpr int TFT_MISO  = 19;
constexpr int TFT_RST   = 4;
constexpr int TFT_DC    = 2;
constexpr int TFT1_CS   = 5;
constexpr int TFT2_CS   = 15;
constexpr int TFT3_CS   = 21;

void setup() {
  pinMode(TFT_RST, OUTPUT);
  pinMode(TFT_DC, OUTPUT);
  pinMode(TFT1_CS, OUTPUT);
  pinMode(TFT2_CS, OUTPUT);
  pinMode(TFT3_CS, OUTPUT);

  digitalWrite(TFT1_CS, HIGH);
  digitalWrite(TFT2_CS, HIGH);
  digitalWrite(TFT3_CS, HIGH);

  digitalWrite(TFT_RST, LOW);
  delay(20);
  digitalWrite(TFT_RST, HIGH);
  delay(120);

  SPI.begin(TFT_SCK, TFT_MISO, TFT_MOSI);
}

void loop() {}

1. Set all three CS pins high before starting SPI.
2. Pulse the shared display RESET line.
3. Initialize SPI on GPIO18/19/23.
4. Initialize each display one at a time with only its CS asserted.