It most probably refers to three-phase electrical wiring (installation), which is typically used to power 3-phase electric motors, traditionally and massively used in industrial applications, but not much in household applications these days. Most older houses and all large buildings have them, but flats in housing blocks typically only have single-phase wiring installations. Virtually all home appliances, from mixers to grass cutters, are typically powered by single-phase motors these days, which is more practical and inherently safer (an unprotected 3-phase motor will stop and may burn quickly, if 1 of the 3 phases trips for any reason and the motor is powered by just the other two).