No worries, nice question actually
Main difference is testnet3 will add test safecoin and apps. Initially data will be dropped and API’s may change slightly. Near end of testnet3 data may stay and safecoin may also stay in place. So basic answer is testnet3 builds all components for beta.
Beta still will not guarantee safecoin (no conversion yet) but will stabilise all API’s and data (we will not drop data in beta). By then we will have carried out a number of tests and hopefully had a lot of community audits etc. (we will be awarding security related bounties for exploits in code/algorithms).
tl;dr testnet3 add all the stuff and safecoin, beta confirm and safecoin go live and maidsafecoin cut over.