Hi dear,

           I am here to help you with your query. 315 marks are very less for the Unreserved category to get a seat in Mumbai government dental college under both state quota and  All India Quota. To get under state Quota at least you have to score 400+ and 500+ for All India Quota as per 2018 cutoffs. 2018 cutoffs for different Mumbai government dental colleges for UR category is as follows

                                                                                      AIR cutoff marks

Nair Hospital Dental College, Mumbai                                        519

Government Dental College and Hospital, Mumbai                      522

You can try for some other private colleges in other states. But Never lose hopes there will be so many other doors open for you. Go for them.
All the best dear.
Thank you.