The user has a balance in each of the presented currencies. He may ask to convert part of the balance from one currency to another. (3 currencies in total)

Currency converter

namespace MyCode
{
    internal class Program
    {
        static void Main(string[] args)
        {
            const string CommandExit = "0";
            const string CommandUSDtoRUB = "1";
            const string CommandUSDtoEUR = "2";
            const string CommandRUBtoUSD = "3";
            const string CommandRUBtoEUR = "4";
            const string CommandEURtoUSD = "5";
            const string CommandEURtoRUB = "6";

            float balanceUSD;
            float balanceRUB;
            float balanceEUR;
            float rubToUSD = 0.0143f;
            float rubToEUR = 0.0133f;
            float usdToEUR = 0.9321f;
            float exchanger;
            string enteredCode;
            bool isActive = true;

            Console.WriteLine("Enter your USD balance: ");
            balanceUSD = float.Parse(Console.ReadLine());

            Console.WriteLine("Enter your RUB balance: ");
            balanceRUB = float.Parse(Console.ReadLine());

            Console.WriteLine("Enter your EUR balance: ");
            balanceEUR = float.Parse(Console.ReadLine());

            while (isActive)
            {
                Console.WriteLine($"\nYour balances are:  {balanceUSD} USD, {balanceRUB} RUB, {balanceEUR} EUR");
                Console.WriteLine("Enter number according your need: \n" +
                    $"{CommandUSDtoRUB} - USD to RUB\n" +
                    $"{CommandUSDtoEUR} - USD to EUR\n" +
                    $"{CommandRUBtoUSD} - RUB to USD\n" +
                    $"{CommandRUBtoEUR} - RUB to EUR\n" +
                    $"{CommandEURtoUSD} - EUR to USD\n" +
                    $"{CommandEURtoRUB} - EUR to RUB\n" +
                    $"{CommandExit} - exit\n");

                enteredCode = Console.ReadLine();
                
                switch (enteredCode)
                {
                    case CommandExit:
                        Console.WriteLine("Exit");
                        isActive = false;
                        break;

                    case CommandUSDtoRUB:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());
                        
                        if (exchanger <= balanceUSD)
                        {
                            balanceRUB += exchanger / rubToUSD;
                            balanceUSD -= exchanger;
                        }

                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    case CommandUSDtoEUR:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());
                        
                        if (exchanger <= balanceUSD)
                        {
                            balanceEUR += exchanger * usdToEUR;
                            balanceUSD -= exchanger;
                        }

                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    case CommandRUBtoUSD:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());
                        
                        if (exchanger <= balanceRUB)
                        {
                            balanceUSD += exchanger * rubToUSD;
                            balanceRUB -= exchanger;
                            break;
                        }

                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    case CommandRUBtoEUR:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());

                        if (exchanger <= balanceRUB)
                        {
                            balanceEUR += exchanger * rubToEUR;
                            balanceRUB -= exchanger;
                            break;
                        }

                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    case CommandEURtoUSD:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());

                        if (exchanger <= balanceEUR)
                        {
                            balanceUSD += exchanger / usdToEUR;
                            balanceEUR -= exchanger;
                            break;
                        }

                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    case CommandEURtoRUB:
                        Console.WriteLine("Enter amount to exchange:");
                        exchanger = float.Parse(Console.ReadLine());

                        if (exchanger <= balanceEUR)
                        {
                            balanceRUB += exchanger / rubToEUR;
                            balanceEUR -= exchanger;
                            break;
                        }
                        
                        Console.WriteLine("Error, you do not have required amount of money!\n");
                        break;

                    default:
                        Console.WriteLine("Error");
                        break;
                }
            }
        }
    }
}