It might be simpler to create a table of related records where you create a record with the next status each time the status change. Then On Create auto enter settings on a time stamp field can capture the current date and time in a timestamp field.
Using a single record with a radio button formatted single status field would require a script performed by the OnObjectModify trigger you'd set on your radio button field. The script would use the value of the radio button field to determine which field to update with get ( currentTimeStamp ).
Why not set an auto-enter calculation for each timestamp? Say, for Timestamp3 (the one that logs when option 3 was selected), use an auto-enter calc of "If ( job_status = 3; Get ( CurrentTimestamp ) )". Make sure to specify that the auto-enter calc shouldn't override existing values.
Edit: You could also condense this functionality into a single repeating timestamp field using the following auto-enter calc: "If ( Extend ( job_status ) = Get ( CalculationRepetitionNumber ); Get ( CurrentTimeStamp ) )".