The 3 free minutes feature applies to any chat session you have with an advisor for the very first time. After 3 minutes, the session will automatically switch into "Hire" mode and you will be charged for each additional minute according to the advisors' per-minute fee. If you decide to contact the same advisor a second time, your session will go into "Hire" mode the moment your advisor starts typing.
Please note that when you start your session there is a very clear note in the chat suite which states that the session will automatically go into hire after the 3 free minutes have elapsed.
Be advised that you have the "Hang up" button if for any reason you do not wish to continue with the session.