uses
  ...
  , Windows
  , ...
  ;

type
  TMouseBtnType = (mbLeft, mbMiddle, mbRight);

const
  MOUSE_BTN_VKEYS: Array [TMouseBtnType] of Integer = (VK_LBUTTON, VK_MBUTTON, VK_RBUTTON);

(* This function returns true when the specified mouse button is pressed *)
function IsMouseBtnDown(const AMouseBtn: TMouseBtnType): Boolean;
begin
  Result := GetAsyncKeyState(MOUSE_BTN_VKEYS[AMouseBtn])
            AND $8000 <> 0;
end;


(* This function returns true when any of the mouse button is pressed *)
function IsMouseBtnDown: Boolean;
begin
  Result := (GetAsyncKeyState(VK_LBUTTON)
             OR GetAsyncKeyState(VK_MBUTTON)
             OR GetAsyncKeyState(VK_RBUTTON)
             )
            AND $8000 <> 0;
end;