SSIS Transaction Level for Single Task
SSIS Transaction Level

The default transaction level for all tasks and containers is Supported which means they will enlist in an transaction if available but they will not create one.. 

A Required settings will start a transaction if one does not exist and enlist in an existing transaction.

NotSupported indicates that the Executable/Container will ignore any existing transactions which may result in deadlocks depending on your design.

Your case

You have 3 Tasks and the desire is to have the third item behave in an atomic way. The answer is, partially, that you'll want to set the Transaction option to Required. Partially because you can commit throughout your data load. In an OLE DB Destination, if you have a non-zero commit size then the committed data is fully committed. A rollback isn't going to pull that data back out.