Where to go from Lucca for 3 days before flying out of Milan?

Do you need to deduct a night to be in Milan before your flight out, or is that accounted for? That might change the calculus!
If your flight is late afternoon, I would feel fine coming from the lakes for an outbound flight, but for an early morning flight I would want to be in Milan.
If it's two nights, I would be tempted to add them to Lucca and do some day trips before heading to Milan. I don't know the lakes area, but with three full days it sounds like a great option.

When I saw your title, before i read your post, my first thought was Cortona. But since your itinerary has that covered, then add Bologna to your list. It’s a bit more geographical zig-zagging across Italy, but if you haven’t yet been to the gastronomic capital of Italy, then you should.

While Bologna is the 7th biggest city in Italy, pop. almost 400,000, the old town, within what remains of the old walls is a compact, mostly pedestrians-only place, and is walking-friendly to restaurants, the train/bus station, museums, several major churches, small parks, twin medieval towers, and other sights, including all the porticoes.

Be aware that ~80,000 people descend upon the tiny historical center of Lucca at the end of October each year for the largest comics convention in Europe — Lucca Comics and Games. Festival-related construction takes over the piazzas starting in early to mid October. So as lovely as Lucca is, despite its now being very much on the tourist map, it won’t have its usual charm. Then again, maybe you are comics enthusiasts and this is why you’ve put Lucca on your itinerary for late October!

You might really like Ferrara --- less than 3 hours by train from Lucca and also (with the right train) less than 3 hours from Ferrara to MXP (MIlan airport). Far fewer tourists than Lucca and good restaurants and things to do and see.

Here's the part about Ferrara from my December 2023 trip report:

My husband signed up for a different kind of cooking class and had a terrific time with a group of friends who cook together at one of their apartments — I joined them for dessert and they switched to English for me: https://www.tripadvisor.com/AttractionProductReview-g194760-d26337067-Private_Cooking_Class_of_Ferrarese_cuisine-Ferrara_Province_of_Ferrara_Emilia_Roma.html

Palazzo Schifanoia frescoes cover the walls in a huge room and were fantastic in every sense of the word: https://smarthistory.org/sala-dei-mesi-at-palazzo-schifanoia/ And nobody was there — we had the room all to ourselves and could walk around pointing and exclaiming at the details. I think that even a person with no interest in art would like to see these frescoes.

Casa Romei had some beautiful frescoes in a 14th century merchant's house. If you have to pick one, though, see Schifanoia.

The Museo della Cattedrale had cool sculptures of the months and the activities done in each month.

https://notaboutthemiles.com/things-to-do-in-ferrara/

Monastero di Sant’Antonio. Three 14th century chapels covered in school-of-Giotto frescoes with the kind of uncommon details I like to see (such as a D.I.Y. Jesus climbing a ladder up to his cross). My husband has had his fill of medieval and Renaissance art, but this monastery visit turned out to be one of his favorite experiences in Italy. You ring the monastery bell and ask to visit the frescoes. A little nun who is allowed to have contact with the public whispers her patter throughout the tour, which includes the dripping tomb of a nun (the holy water is collected in bottles and used for I-don’t-know-what). No photos. At the end, donate or buy some postcards. See: Monastero di Sant'Antonio in Polesine — Ferrara Terra e Acqua
https://iviaggidiraffaella.blogspot.com/2017/08/ferrara-gli-affreschi-del-monastero-di.html

We enjoyed eating at I Tri Scalin (recommended by my husband’s cooking class buddies for traditional food) and Trattoria da Noemi.

Ferrara has an extensive Jewish history and heritage if that is of interest to you.

Last October, before flying out of Milan, I spent 3 nights in Varenna on Lake Como with a final night in Milan. For one of the days in Vaarenna, I took the Bernina route train to Pontresina Switzerland and back for oustanding scenery. I also recommend Bologna as previously mentioned. You might also enjoy Parma which is on the way from Lucca to Milan. Ravenna or Padua are maybe a little out of the way but very doable by train. I second the recommendation to spend the final night in Milan before flying out.

You may consider going to Lake Maggiore,
visit the beautiful Borromean Islands, then take the direct Alibus bus to Malpensa.
Stresa is popular with many here but I stayed in Baveno (4 Km North of Stresa)

Enjoy your trip!