[noob]

Lo tengo y funciona, muchas gracias.

Form1:

Código Delphi [-]
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, Unit2;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    Clase2: cClase2;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  Clase2 := cClase2.Create;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Clase2.anadir(Edit1.Text);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  Clase2.mostrar;
end;

end.

Unit2:

Código Delphi [-]
unit Unit2;

interface

type
  cClase2 = class
              private
              public
                procedure anadir(Mensaje: string);
                procedure mostrar;
            end;

implementation

uses Unit3, Dialogs;

procedure cClase2.anadir(Mensaje: string);
begin
  with Clase3 do
    Contenido := Contenido + Mensaje;
end;

procedure cClase2.mostrar;
begin
  ShowMessage(Clase3.Contenido);
end;

end.

Unit3:

Código Delphi [-]
unit Unit3;

interface

type
  cClase3 = class
              private
              public
                Contenido: string;
            end;

var
  Clase3: cClase3;

implementation

initialization

  Clase3 := cClase3.Create;

finalization

  Clase3.Free;

end.