Continuously Measure 3-Phase Motor Current
You need a current transformer (or three).  



If you connect a resistor to the transformer's secondary winding a current through the sensed wire will cause a voltage across the resistor. The voltage is proportional to the current so will be a sine wave. 

If you'd rather like a DC output proportional to the current you can use an i-Snail-V current transformer. It's self-powered and the i-Snail-V10 version for instance gives you 5 V out for 10 A. Other versions with other sensitivities exist.