[NumLock]

Код:

unit Unit1;

interface

uses
  Unit2, Unit3,
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  Caption:=IntToStr(do_something2)+' '+do_something3;
end;

end.

Код:

unit Unit2;

interface

var
  Bla_Unit2: String = 'ok';

function do_something2: Integer;

implementation

uses
  Unit3;

function do_something2: Integer;
begin
  Result:=Bla_Unit3;
end;

end.

Код:

unit Unit3;

interface

var
  Bla_Unit3: Integer = 255;

function do_something3: String;
  
implementation

uses
  Unit2;

function do_something3: String;
begin
  Result:=Bla_Unit2;
end;

end.

кто главный Unit2 или Unit3?)
Unit1 не в счет - в нем только главная форма