Declaration of MulticastDelegate which is default base class of delegates should include 

public string Invoke(bool a, bool b, bool c);//which has to match with delegate function signature

public IAsyncResult BeginInvoke(bool a, bool b, bool c,AsyncCallback cb, object state);//Above plus callback.

public string EndInvoke(IAsyncResult result); //get the results.

 

but if the delegate includes ref or out method the EndInvoke method has to include the them.