There's always a risk with a connection on two different reservations/PNRs. The real risk is your DL flight arriving late. I would be okay with having a 3 hour connection and have done un-attached connections in less, but that's just me. I've never spent more than a half-hour waiting at EU passport control at FCO and usually it's a lot less.

Delta uses T3 while ITA used T1. Landside, FCO is a surprisingly compact airport. After clearing passport control, go upstairs to the departure level, exit the terminal and turn left. It will be about a five minute walk to the ITA check-in counters in T1.