[NumLock]

Код:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TFormThread = class(TThread)
  private
    AForm: TForm;
    procedure ShowMessage;
  protected
    procedure Execute; override;
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses Unit2, Unit3;

{$R *.dfm}

{ TFormThread }

procedure TFormThread.Execute;
begin
  AForm:=Form2;
  Synchronize(ShowMessage);
  Form3.Memo1.Text:=Form2.Edit1.Text;
  AForm:=Form3;
  Synchronize(ShowMessage);
  Windows.Beep(1000, 100); { чтобы услышать когда поток закончился }
end;

procedure TFormThread.ShowMessage;
begin
  AForm.ShowModal;
end;

{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject);
begin
  TFormThread.Create(False);
end;

end.

полный пример