procedure TForm1.Button1Click(Sender: TObject);
var
  slDest, slSrc:TStringList;
  sALL,sA, sB, sC:String;
begin
  sA:='C:\aa.txt';
  sB:='C:\bb.txt';
  sC:='C:\cc.txt';
  sALL:='C:\all.txt';
  slDest:=TStringList.Create;
  slDest.LoadFromFile(sA);
  slSrc:=TStringList.Create;
  slSrc.LoadFromFile(sB);
  slDest.AddStrings(slSrc);
  slSrc.LoadFromFile(sC);
  slDest.AddStrings(slSrc);
  slDest.SaveToFile(sALL);
end;