C# delegate+event ...question?

If I understand correctly (it's still not too clear), you are trying to coordinate Forms A, B and C so that when one is changed, the information from those forms appears in all of the forms. Correct me if I got that wrong.

If that's what you want to do, then you can create a delegate (in Form A and B?) and pass that to the other form(s). Then when the other forms change, in their event handler for whatever event that occurs when something of interest changed, you call the delegate with the required information that will update the other forms that defined the delegate. In the delegate(s), you update the information in the respective form(s).

For example (please check the syntax, this is just pseudo-code):

In Form A:

// Implement the delegate function to update A when called
void MyFormADelegateHandler(int MyParameterFromB)
{
  // Update Form A with info from MyParameterFromB
  ...
}

  ...
  // Wherever you load Form B, set the delegate in B to call your function
  frmB = new FormB(); // etc.
  frmB.ADelegate = MyFormADelegateHandler;
  frmB.Show();
  ...

Then in Form B:

// Declare the delegate type
public delegate void FormADelegate(int MyParameterFromB);

// Declare a variable to hold the pointer to the delegate function
// I set this to null so it can fail gracefully if no delegate is provided
// That's just how I do it. It may not be necessary, but it's clearer for me.
  public FormADelegate ADelegate = null;

And in the event handler for whatever changed in Form B:

...
// Handle Form B event for whatever changed in Form B
...
// Then call the function passed from Form A
if (ADelegate != null)
{
  ADelegate(MyParameter);
}
...

There might be other ways to do the same thing, but hopefully that should be enough to get you started.