unit Unit1;

interface

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

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

var
  Form1: TForm1;

implementation
[color=#0000FF]
uses Unit2, Unit3;

{$R *.dfm}
var
    frmActive:TForm;

procedure TForm1.Button1Click(Sender: TObject);
begin
    if assigned(frmActive) then frmActive.Close;
    form2.Show;
    frmActive:=form2;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
    if assigned(frmActive) then frmActive.Close;
    form3.Show;
    frmActive:=form3;
end;
[/color]
end.

同意楼上