I have scoliosis, pectus excavatum, and asymmetrically flared ribs. Could wearing a corset help my posture? Are there other non surgical options?

Yes. While a corset may help one's posture, it will not correct or stop the problem of one's bones changing shapes or positions. Nonsurgical options for such conditions may include a body brace that can be worn all day to slow deformation, depending on the severity of symptoms. An orthopedic doctor specializing in spine and scoliosis therapy can evaluate and offer treatment advice.
Maybe. Pectus is sometimes associated with other issues of the spine as in you case. Flaring of the ribs is commonlky associated with pectus excavatum. In some instances a combined surgical approach by orthopedis and pediatric surgery can treat both issues.