LH should operate from terminal 3, along with the other *A airlines.
this is also what the FCO website says:
https://www.fiumicinoairport.net/ter...terminal.shtml
seems the airport website is up to date, just left in SK out of terminal 3 as well, but SK said it should be terminal 1. I guess they moved recently.
by the way, the lounge is really nice with decent food.