You have to pass the Series 3 first, then to register with the CFTC and to become member of the NFA ($750/year), and to draft your first disclosure documents describing your strategies/investments to be a legit CTA in the US, trading futures, options and FX on behalf of clients.
You may not need to register (there are several exemptions from registration), but to become a CTA who manages real money and wants to promote himself as a CTA, you would need to register with the CFTC/NFA.