Medical device maker Boston Scientific said it swung to a third-quarter loss, as legal fees and restructuring expenses weighed down already-sluggish sales of the company’s implants. The Natick company adjusted its full-year guidance. It now expects adjusted earnings of 63 to 66 cents per share. Its prior forecast was 62 to 68 cents. For the quarter, it lost $725 million, or 52 cents per share, compared with a profit of $142 million, or 9 cents, a year earlier.