Are you sure it is not about 3 KW?
I would simply run away as far as possible from a 3kV power supply.
If it is about current lines (wires that transport electricity from one geographical area to another), then it turns to measuring electromagnetic field and not the current as such - IMAO. The accuracy of indirect measuring the current in such circumstances (high voltage, Arduino platform) is questionable- again IMAO.
I would also like to stress that Arduino is a learning platform. So, perhaps starting by measuring smaller electromagnetic fields (like the ones from the house power supply) may be a fun (and reasonably safe) way to learn.