Message return function

Hello, I have a few small knowledge issues.

I would like to create a message return and acknowledgment function. Example, here is my program.

//#include <RadioHead.h>
//#include <RHSoftwareSPI.h> //interface de Bus SPI du module LoRa
#include <SPI.h>
#include <RH_RF95.h> // Bibliothèque RadioHead pour les comms LoRa

#define RFM95_CS 8
#define RFM95_RST 4
#define RFM95_INT 3

RH_RF95 rf95(RFM95_CS, RFM95_INT);

uint8_t buf[255];
uint8_t len = sizeof(buf);
//char message[11];
int i;

void setup () {

Serial.begin(115200);

rf95.init();
rf95.setModemConfig(RH_RF95::Bw125Cr45Sf128); // largeur de bande = 125kHz Code rate 4/5 Spreading factor = 2^7
rf95.setFrequency(868.1); //868.1,868.2 ou 868.3
rf95.setTxPower(20,false);
pinMode(LED_BUILTIN,OUTPUT);
}

void loop () {

String message="TP M2 IOT#";
message+=i;
i++;
message.toCharArray((char*)buf,message.length()+1);
buf[message.length()+1]=0;
delay(100);
digitalWrite(LED_BUILTIN,HIGH); // Eclairage de la LED pendant l'émission
rf95.send((uint8_t*)buf,message.length()+2);
rf95.waitPacketSent();
digitalWrite(LED_BUILTIN,LOW); //Eclairage de la LED éteins quand il n'y a pas d'émission
Serial.print("Message envoye:");
Serial.println(message);
//delay(2000);

if(rf95.waitAvailableTimeout(6000)){ //Attente d'une réception pendant 4 secondes
if(rf95.recv(buf,&len)){
Serial.println((char*)buf);
Serial.print("RSSI:");
Serial.println(rf95.lastRssi(), DEC);
Serial.print("SNR:");
Serial.println(rf95.lastSNR(), DEC);
}
}
else Serial.println("Aucune Reception");
}

I have 3 microcontrollers, 1 which sends a message, the 2nd which receives it and which resends the received message to the 3rd (because the 1st microcontrollers is to far from the 3rd). The 3rd must send an acknowledgment message to the 2nd. How to do ? Thank you !

The easier you make it to read and copy the code the more likely it is that you will get help

Please follow the advice given in the link below when posting code , use code tags and post the code here

If your second microcontroller is receiving a message (from #1) and then sending a message (to #3) then you should use the same principle for your third controller when it receives a message (from #2) to send a message (to #2).

From which microcontroller is the code you posted ? I suggest posting code from both #2 and #3.