Vayeira

Why does Hashem need to send a Malach to tell Avraham that Sara wil have a son?Hashem just told him 3 days earlier in17,16+19! It can't either be so Sara finds out because then the logical thing to do would be to have Avraham pass on the message to Sara, and then she probably would have believed him as he is a Navi Muchzak(Established)!