Having lived there for 5 years, we liked that late fall time a lot. It was cool enough for a jacket in the morning and evening, but often shirt-sleeves weather during the day in Rome. So mild temps and far better than the heat of summer.
Can it rain? Sure. But it seldom is an all-day soaker. When it does rain, if you are in a city like Rome or Florence, you have lots of indoor choices to entertain yourselves. Venice is a little more challenging in rain, but still lovely and again, in many many visits there in all but the height of summer (we usually go there between October and December), it has never drowned out our enjoyment. And we love a nice off-season discount. We went to Florence one January and paid half-price at a place we would otherwise not have looked at due to price. Although it rained a bit, the museums were uncrowded.
How many nights will you have? V-F-R takes at least 10 nights, IMO, 14 is better. A smaller town option might be Orvieto as it is on the train line between Florence and Rome. Try to fly into one city and out of the other (i.e., into Venice and out of Rome) to minimize backtracking.