why longer run time = more MB in RAM

Question

my app have a problem. Longer run time = more MB in RAM. It start after i tryed to using Threads. Im using 3x Tread with timer. Threads im using because im using httpget, and without it GUI freeze always when it start call httpget that download file about 150 byte. Timer interval can be set at 5 - 300 seconds most of times is used 5s. So every timer time im calling 3x Thread and in each Thread im calling httpget. But app eating memory maybe im doing something wrong with threads? Maybe they stay "open" somehow?

var
  a, b, c: string;

type
  TThread_1 = class(TThread)
    private
      FVar_1: string;

      procedure Update_1;
    protected
      procedure Execute; override;
    end;

  TThread_2 = class(TThread)
    private
      FVar_2: string;

      procedure Update_2;
    protected
      procedure Execute; override;
    end;

  TThread_3 = class(TThread)
    private
      FVar_3: string;

      procedure Update_3;
    protected
      procedure Execute; override;
    end;


procedure TThread_1.Execute;
begin
  FVar_1 := HttpGet('url', 'a');
  Synchronize(Update_1);
end;

procedure TThread_1.Update_1;
begin
  a := FVar_1;
end;

procedure TThread_2.Execute;
begin
  FVar_2 := HttpGet('url', 'b');
  Synchronize(Update_2);
end;

procedure TThread_2.Update_2;
begin
  b := FVar_2;
end;

procedure TThread_3.Execute;
begin
  FVar_3 := HttpGet('url', 'c');
  Synchronize(Update_3);
end;

procedure TThread_3.Update_3;
begin
  c := FVar_3;
end;


procedure TForm1.Timer_thread(Sender: TObject);
var
  t1 : TThread_1;
  t2 : TThread_2;
  t3 : TThread_3;
begin
  if lb_1.IsChecked = True then
    begin
      t1 := TThread_1.Create(true);
      try
        t1.FreeOnTerminate := true;
      finally
        t1.Start;
      end;
    end;

  if lb_2.IsChecked = True then
    begin
      t2 := TThread_2.Create(true);
      try
        t2.FreeOnTerminate := true;
      finally
        t2.Start;
      end;
    end;

  if lb_3.IsChecked = True then
    begin
      t3 := TThread_3.Create(true);
      try
        t3.FreeOnTerminate := true;
      finally
        t3.Start;
      end;
    end;
end;

function HttpGet(const url: string; const tip: string): string;
var
  HTTP: TIdHTTp;
begin
  HTTP := TIdHTTP.Create(nil);
  HTTP.Request.UserAgent := 'Mozilla/6.0 (compatible; Delphi)';
  try
    try
      Result:=http.Get(url);
    except
      on E : Exception do
        begin
          if Pos('Error resolving Address', E.Message) <> 0 then
            begin
              if tip = '1' then Form1.lb_1.Text:='1';
              if tip = '2' then Form1.lb_2.Text:='2';
              if tip = '3' then Form1.lb_3.Text:='2';
            end;
        end;
    end;
  finally
    FreeAndNil (http);
  end;
end;

Edited code but still eating memory (1MB / 1 minute - timer interval 5 seconds), if timer turn off it stop eat memory:

procedure TForm1.Timer_thread(Sender: TObject);
var
  t1 : TThread;
  t2 : TThread;
  t3 : TThread;

begin
  if lb_1.IsChecked = True then
    begin
      t1:= TThread.CreateAnonymousThread(
        procedure
        var s: string;
          begin
            s := HttpGet('url_a', 'a');
            TThread.Synchronize(nil,
            procedure
              begin
                a := s;
              end);
          end);
      t1.Start;
    end;

  if lb_2.IsChecked = True then
    begin
      t2:= TThread.CreateAnonymousThread(
        procedure
        var s: string;
          begin
            s := HttpGet('url_b', 'b');
            TThread.Synchronize(nil,
            procedure
              begin
                b := s;
              end);
          end);
      t2.Start;
    end;

  if lb_3.IsChecked = True then
    begin
      t3:= TThread.CreateAnonymousThread(
        procedure
        var s: string;
          begin
            s := HttpGet('c', 'c');
            TThread.Synchronize(nil,
            procedure
              begin
                c := s;
              end);
          end);
      t3.Start;
    end;
end;