SQL is a programming language to manage data and data definition in RDBMS.
Example: "SELECT * FROM users WHERE lastname = 'Agrawal';"
MySql is a relational database management systems (RDBMS), which hold databases that are based on relations. (Other RDBMS are e.g. Postgresql or MS SQL Server)
SQLite is also a RDBMS but a lightweight one, it doesn't require it's own process, clustering or user management unlike MySQL or others RDBMS. SQLite consist of a single file and a library to make your application interact with it. The typical example is storing bookmarks on a web browser, or minor database in mobile apps.
SQLite3 is the version 3 of SQLite which provides - more compact format for database files, manifest typing and BLOB support, support for both UTF-8 and UTF-16 text, user-defined text collating sequences, 64-bit ROWIDs and Improved Concurrency.