3kW continuous would be an RMS current of (I make it) 28A into 4 ohms. Also it's 110V, so you need well- insulated cable. RS in Britain do 6 square mm twin cable, but at £100 for 25 metres it's expensive. They do 4 sq mm a about a reel, that should do for 2kW into 4 ohm.

Remember that only heavy metal bands ever pump out full power for more than a few seconds at a time, and they don't mind their gear smoking, so you can probably get away with cheap, readily available 2.5 sq mm flex even with a largish stack.

At these currenst the connectors need to be substantial and well maintained, or you'll set something on fire.