Two investor rating agencies have given MaineHealth strong marks for its financial stability, even after accounting for $300 million in borrowing to finance a hospital expansion. The ratings from Moody’s and Standard & Poor’s puts the Portland-based network at No. 3 in New England for financial soundness, after the parent companies of first-place Massachusetts General and second place Yale-New Haven hospitals, according to a statement from MaineHealth. “This is big news for us,” said John Porter, MaineHealth communications director.