Try:

#define Pin_A D6
#define Pin_B D7
#define Pin_C D8

Also remove these delays.

pinMode(Pin_A, OUTPUT);
  pinMode(Pin_B, OUTPUT);
  pinMode(Pin_C, OUTPUT);

Follows all the code, anyone have some advice?

// RemoteXY select connection mode and include library
#define REMOTEXY_MODE__ESP8266WIFI_LIB
#include <ESP8266WiFi.h>

#include <RemoteXY.h>

// RemoteXY connection settings
#define REMOTEXY_WIFI_SSID "FRITZ!Box 7490"
#define REMOTEXY_WIFI_PASSWORD "32189651531398700841"
#define REMOTEXY_SERVER_PORT 6377

#define Pin_A 6
#define Pin_B 7
#define Pin_C 8

// RemoteXY configurate 
#pragma pack(push, 1)
uint8_t RemoteXY_CONF[] =
  { 255,1,0,0,0,11,0,8,13,1,
  3,3,29,36,8,22,2,26 };
 
// this structure defines all the variables of your control interface
struct {

    // input variable
  uint8_t select_1; // =0 if select position A, =1 if position B, =2 if position C, ...

    // other variable
  uint8_t connect_flag;  // =1 if wire connected, else =0

} RemoteXY;
#pragma pack(pop)

/////////////////////////////////////////////
//           END RemoteXY include          //
/////////////////////////////////////////////

void setup()
{
  RemoteXY_Init ();

  pinMode(Pin_A, OUTPUT);
  pinMode(Pin_B, OUTPUT);
  pinMode(Pin_C, OUTPUT);
 
  // TODO you setup code

 
}

void loop()
{
  RemoteXY_Handler ();
 
switch (RemoteXY.select_1) {
  case 0:
    digitalWrite(Pin_B, LOW);   
    digitalWrite(Pin_C, LOW);   
    digitalWrite(Pin_A, HIGH);
    delay(1000);
     
    break;
  case 1:
  digitalWrite(Pin_A, LOW);   
    digitalWrite(Pin_C, LOW); 
    digitalWrite(Pin_B, HIGH);
    delay(1000);
    break;
  case 2:
    digitalWrite(Pin_A, LOW);   
    digitalWrite(Pin_B, LOW); 
    digitalWrite(Pin_C, HIGH);
    delay(1000);

    break;
}
 
  // TODO you loop code
  // use the RemoteXY structure for data transfer

}