Get all the possible combinations

I need to get all the possible combinations of coins available to pay a ceiling price, but limiting the amount of coins to 3 units maximum of each value (to pay € 10 it does not cost 10 coins of € 1).

With the following code I get a combination limited to 3 units per coin, but how can I modify the code to print all possible combinations?

class Program
{
    static int precio = 450;
    static void Main(string[] args)
    {
        Moneda[] c = new Moneda[] { new Moneda(300, 3), new Moneda(150, 3), new Moneda(120, 3) ,
                            new Moneda(100, 3), new Moneda(75, 3), new Moneda(60, 3),
                            new Moneda(36, 3), new Moneda(20, 3), new Moneda(18, 3)};

        for (int i = 0; i < c.Length; i++)
        {
            precio -= c[i].precio2(precio);
            Console.WriteLine(c[i].ToString());
        }
        Console.ReadLine();
    }
}
class Moneda
{
    private int precio;
    private int counted;
    private int maxNo;

    public Moneda(int Monedaprecio, int MonedaMaxNo)
    {
        this.precio = Monedaprecio;
        this.maxNo = MonedaMaxNo;
        this.counted = 0;
    }
    public int precio2(int Precio)
    {
        int Num = Precio / precio;
        if (maxNo == 0)
            return 0;
        if (maxNo != -1)
            if (Num > this.maxNo - this.counted)
                Num = maxNo;
        this.counted += Num;
        return Num * precio;
    }
    public override string ToString()
    {
        return string.Format("{0} x {1} (max {2}) ", this.precio.ToString(), this.counted.ToString(), this.maxNo.ToString());
    }
}

It must be simple, but I am a beginner programmer and I do not see how to do it. Thanks.