A current transformer is overkill, there are relatively cheap clip-round Hall-effect current transducers available with 5V supply and analog outputs centred on Vcc/2. Many household electricity monitors use them and some allow you to add more to handle multple phases. For instance the ones used in these: http://www.efergy.com/Products/efergy-Shop-Energy-Monitors/EFERGY/Elite-Wireless-Monitor/pid-184321.aspx#accessories
Measuring mains voltages is much more problematic - the use of a small step-down transformer is the right sort of thing, but again protection is needed against line spikes and the AC level needs shifting suitable for the A/D conversion.
3 phases of current and voltage means 6 values to sample many times per mains cycle, squared, averaged and square-rooted to give RMS readings... I think you can get about 6 analogRead's per millisecond which should be enough.