Why does my kitchen faucet have 3 pipes?
Because these local water heaters are often open systems, i.e. the increased pressure from heated water must not damage the installation. The pressure of the water net is only up to the faucet valve. After that valve it is in contact with the normal air (pressure) and flows to the heater, back to the faucet valve where it can be mixed with the cold water. Many countries have rules that heated water must not flow back to the cold water net, a backflow preventer is in the incoming line of the heater. That is why the pressure can not be decreased upstream, only downstream. It is dangerous to use a normal faucet with those heaters, since the hot water tank could burst spreading hot water into the room.