Microsoft Equation 3.0 (.xxx)
The XXX file extension, associated with Microsoft Equation 3.0, is used for storing equation files created in the Equation Editor of Microsoft Word. Historically, Microsoft Equation 3.0 was a popular tool for inserting mathematical equations into documents, but it was removed from Office 365 and later versions due to security vulnerabilities. Users can now edit existing XXX files using alternative software like MathType.