C#

I have 3 classes overriding a function of base abstract class A. The child classes are 1, 2 & 3. Child classes override function "public abstract void hello(var1, var2)".

Following is what I want to implement:
abstract class A
{
public abstract void hello(var1, var2); // This will be only for class 1 & class 2 (Existing function)
public abstract void hello(var1, var2, var3); // Want to make this available for class 3 only. (Want to add this new function)
}


** Currently the above code throws error and wants me to implement the new inherited abstract function in class 1 & 2.

public abstract class Factory
    {
        private readonly object _instanceLock = new object();
        private Factory _instance = null;

        internal Factory GetInstance(Type t)
        {
            if (_instance != null) return _instance;
            if(t != typeof(Factory)) throw new InvalidOperationException("Factory can't created an instance not derived from " + typeof(Factory).FullName);

            lock (_instanceLock)
            {
                if (_instance != null) return _instance;
                Thread.MemoryBarrier();
                var assembly = Assembly.GetAssembly(t);
                _instance = (Factory)assembly.CreateInstance(t.FullName);
            }

            return _instance;
        }

        protected Factory() { /* Protected -> Singleton */ }
    }