On a Lenovo, which of the three sound ports should I use to pluf in sound?

Probably "Line In" if from a source like a player or audio system.
For microphone input, use the "Mic" port.
"pluf in sound" might mean to you where to put the speakers or headphones plug. That goes into the "Audio Out"
That is all that can be guessed from only "Lenovo" without a type of computer or a model number.

See owner's manual for your specific model. -- manual is online at lenovo website and model number is on the plate/label on the back