A notification is published in the Official Gazette by the Government in exercise of the power derived from a legislative enactment or provision. When you read “as may be prescribed”, the information published can be in the form of Rules or Notifications. It is part of the Act and binding on all the stakeholders.
A Circular is issued by the Board or Head of the department clarifying, explaining, or interpreting certain provisions. Usually, Circular are binding on the departmental officers. They have to act based on the circulars. But they are not binding on people.
For example – If the CBDT issues a Circular, the tax department has to follow them. The taxpayer can follow if it is to his advantage. If it is against the taxpayers wishes, he needn’t follow them.
A Press Release is an Official announcement by the Government or the Board covering the highlights of the proposal. The press Release is made by Press Information Bureau (pib.gov.in) The PIB is a nodal agency disseminates information to the print, electronic and new media on government plans, policies, program initiatives and achievements.