NIM in C++

I need help trying to decide if the user is entering 3 numbers next to each other. In the game of circular Nim, the user can enter the total number of pieces he/she wants to use, than who goes first. How would i write code to say: if the entered numbers are next to each other (e.g 1 2 3, or in a 15 piece game 14,0,2) than its legal otherwise its not.

See code below. Highlighted is the user function.

Code:

#include<iostream>
#include<string>
#include <iomanip>
#include<cmath>
using namespace std;


void SetParamters(int&, bool&); //prompts user for paramters
void FillBoard(int, char[]);
void PrintBoard(char [], int);
void ComputerTurn(char [], int&, int);
void HumanTurn(char[], int&, int);




int main ()
{
	int pieces,size, piecesSlection=0;
	char GamePieces[20];
	bool  COMPUTER_TURN=false;
	

SetParamters(pieces, COMPUTER_TURN);
size=pieces-1;
FillBoard(pieces, GamePieces);
//PrintBoard(GamePieces, pieces);
do
{
if (COMPUTER_TURN==true)
{
	ComputerTurn(GamePieces, piecesSlection, pieces);
	PrintBoard(GamePieces,pieces);
	COMPUTER_TURN=false;
}
else 
{ 
	HumanTurn(GamePieces, piecesSlection, size);
	PrintBoard(GamePieces,pieces);
	COMPUTER_TURN=true;
}
}while (GamePieces[0]!='_');//needs adjustment


return 0;
}

void SetParamters(int& pieces, bool& COMPUTER_TURN)
{
	char answer;
	cout<<"Welcome To NIM"<<endl<<endl;
	do
	{

	cout<<"How many pieces would you like to start with?"<<endl;
	cin>>pieces;
	if (pieces<5||20<pieces)
	
		cout<<"You can only choose a number between 5 and 20 try again."<<endl<<endl;
	
	}while(pieces<5||20<pieces);

	cout<<"Would you like to go first? (Y/N)";
	cin>>answer;
	if (answer=='y'||answer=='Y')
	{
		cout<<"OK, you can go first."<<endl;
		COMPUTER_TURN=false;
	}
		

	else 
	{
	COMPUTER_TURN=true;
	cout<<"OK, I will go first."<<endl; 
	}
	
		
}
void FillBoard(int pieces, char GamePieces[])
{
	for (int i=0; i<pieces;i++)
	
		GamePieces[i]='o';
	
}
void PrintBoard(char GamePieces[], int pieces)
{
	cout<<endl<<endl<<"HERE IS OUR CURRENT BOARD"<<endl<<endl;
		
	for (int i=0; i<pieces; i++)
		cout<<left<<setw(3)<<i;
	cout<<endl;
	for (int i=0; i<pieces; i++)
		cout<<setw(2)<<GamePieces[i]<<' ';
	cout<<endl;

	
}
void ComputerTurn(char GamePieces[], int& piecesChoice, int pieces)
{
	int computerMove= piecesChoice+1;
	cout<<endl<<endl<<"OK, ITS MY TURN!"<<endl;
	if (computerMove<=pieces&&GamePieces[computerMove]!='_')
	GamePieces[computerMove]='_';
}
void HumanTurn(char GamePieces[],int& piecesSlection, int size)
{
	char answer, count=0;

	cout<<"It's Your turn! Which Pieces Would you like?";
	do{
		cin>>piecesSlection;
		count=count++;
	if (piecesSlection>size||GamePieces[piecesSlection]=='_')
    {
        do
        {
        cout<<"This is an illegal move \n";
        cout<<"Please enter anthor number \n";
        cin>>piecesSlection;
        
        
        }while(piecesSlection>size||GamePieces[piecesSlection]=='_');
    }
	GamePieces[piecesSlection]='_';
	cout<<"Would you like anthor? \n";
	cin>>answer;
	if(answer=='y'&&count<3||answer=='Y'&&count<3)
		cout<<"Please enter your choice \n";
	if(count>2)
		cout<<"You have reached the limit. You are only allowed three."<<endl;
}while(answer=='y'&&count<3||answer=='Y'&&count<3);
}

You should check pairwise differences -- you should get 1, 1, and 2 (in some order). You may have to be willing to change 14 to -1 or so to get the circularity.