Need Help with an assignment

[Chiron]

Write a program that calculates a customer’s monthly bill. It should input customer name, mailing address,
telephone number, the month, which package the customer has purchased, and how many minutes were
used during the month. It should then create a bill that includes the input information and the total amount
due. As an incentive from the mobile phone company to its customers, the program should also display
how much money Package A customers would save if they purchased packages B or C, and how much
money package B customers would save if they purchased package C. If there would be no savings, no
message should be printed.
Input Validation: Be sure the user only selects package A, B, or C.

  #include <iostream>
#include <cmath>
#include <string>


using namespace std;

int main()
{
	//Initialize Variables
	string Cust_Name;
	string Mail_Address;
	double Phone_Num;
	string Month;
	char Plan;
	double Min_Used;
	//Enter Customer Information
	cout << "Please Enter the Customer's Name:" << endl;
	cin >> Cust_Name;
	cout << "Please Enter the Customer's Mailing Address:" << endl;
	cin >> Mail_Address;
	cout << "Please Enter the Customer's Phone Number:" << endl;
	cin >> Phone_Num;
	cout << "Please Enter the Month Being Billed:" << endl;
	cin >> Month;
	cout << "Please Enter the Plan:" << endl;
	cin >> Plan;
		if(Plan != A, Plan != B, Plan != C)
		{
			cout << "Please Enter a Valid Plan:" << endl;
		}
	else cout << "Please Enter the Amount of Minutes Used:" << endl;
	cin >> Min_Used;
	
	system("pause");
	return 0;
}

Im just trying to figure out how to get the input of Plan to only be A, B, or C. Also, how do i add a space character when typing the input for Mail_Address?
Thanks in advance