Microsoft Equation Editor 3.0 is no longer available in any Office Suites. This functionality has been removed by the vendor due to security issues.
Details about this changes are referenced here:
https://support.microsoft.com/en-us/help/4057882/error-when-editing-an-equation-in-office
Microsoft recommends using the built-in equation editing tools instead: https://support.office.com/en-us/article/write-insert-or-change-an-equation-1d01cabc-ceb1-458d-bc70-7f9737722702?ui=en-US&rs=en-US&ad=US#ID0EACAAA=Insert_built-in_equation
Cause
Equation Editor 3.0 is a third-party component built by Design Science (http://www.dessci.com) that was included in many versions of Office, but due to security issues with its implementation, it has been removed. For more information about the security issue, see CVE-2017-11882 | Microsoft Office Memory Corruption Vulnerability.
Resolution
You can insert and edit new equations using the editor built into Office version 2007 or later. For more information about inserting and editing equations, see Write, insert, or change an equation. However, you will not be able to edit existing equations that were inserted using Equation Editor 3.0 in that manner.
Alternatively, the third-party app MathType enables you to edit Equation Editor 3.0 equations without security issues. MathType is now part of the Wiris Suite. You can download a free MathType 30-day trial at: http://www.wiris.com/equation_editor/microsoft