I assume you mean three generations, meaning elderly and less mobile, down to teens or pre-teens. Lake Como is great if you choose one of the towns with a ferry, e.g., Varenna. The older ones can visit the different towns and take journeys by ferry to different areas of the lake, and also just enjoy the view from the balcony or waterside. It tends to be cooler than other areas of Italy, and it has some great food.

For the younger ones, it's on a lake. Varenna has a small beach. They can kayak, motor boat, sail, fish, waterski, wakeboard, go to the large beach at Colico, kitesurf, snorkel, canoe, paraglide, stand up paddle, windsurf, hike up the great trail from Varenna up to the old castle di Vezio, hike in the wooded paths with a guide, take a tram to the top of the mountains, go canyoning, horseback riding, visit archeological sites, dozens of museums, attend the music festival that runs from April through September, catch a fireworks on the lake display.

There's no way to do more than scratch the surface. And you won't suffer from the oppressive heat of Italy in the summer.