The applicant is supposed to mention any previous visa applications on the forms. If the applicant does not, this can be cause for a finding of misrepresentation, which means the application will be refused and the applicant will be banned for two years from applying again.
3 discrepancies from a previous application might also be grounds for a finding of misrepresentation. This would depend on how serious the visa officer feels the differences are, and whether he or she feels they were just honest mistakes or lies.
All you can do about the discrepancies is explain why these details are different. If you have a good reason, just explain and show any proof you have. For the failure to mention the previous visa application, you can try explaining you did not understand the question on the form that asked about previous applications.