If you don't want to use transformers, and I think three small 230V-5V (something like 1VA) transformers for a three-phase system are the cheapest solution, then you have to consider isolation amplifiers ..
For basics on iso-amplifiers try: **broken link removed**
The amplifiers you will be looking at are: AD202, AD210, .., ISO124 ( **broken link removed** ) , and so no ..
Other option would be to get an isolated optocoupler and design a linear amplifier by yourself, but I don't think it is worth the hassle ..
Whichever option you choose, you will need a voltage divider to lover the voltage from 230 (if you have three phase + Neutral configuration), or from 380/440 if you don't have neutral available ..
In both cases you can consider the use of capacitors, instead of resistors, to reduce the voltage; for example, 1µF at 50Hz represents impedance (resistance) of 3.18kΩ, but will not generate heat ..
Regards,
IanP