All three systems server for submitting your requests to different Kaspersky Lab services, for example to Technical Support or Virus Lab.
MyAccount is designed for users of home products and applications for small business, such as Kaspersky Internet Security or Kaspersky Small Office Security.
CompanyAccount is designed for users of products and services for corporate network protection, such as Kaspersky Security Center or Kaspersky Hosted Security.
Personal Cabinet is a legacy system common for all users of Kaspersky Lab products. We are in the process of transferring all home users to MyAccount, and all corporate users to CompanyAccount.