I don't know of any in Venice. That doesn't mean there isn't a gay bar or club there. It's just that that wasn't on my radar when I was there both times. Too much to see and do. That's my favorite city in the world. I would much rather spend the time just walking around and getting lost in the maze of streets while soaking in the atmosphere than sit in a bar. That's just me.
Lake Como is very small and I doubt very, very seriously you'll find a gay bar/club there.
I went to a couple of gay bars in Milan because there wasn't a whole lot to see and do there. However, I wasn't too impressed with the gay bars I went to. I actually had a lot more fun at the straight bars I went to there. It was the opposite of the U.S. The straight bars were loads of fun, happy people, etc. The gay bars were full of cadavers with drinks in hand.
Sorry. Just think those areas are pretty lacking in gay nightlife. Especially compared to other places in Europe. Milan would be your best bet. Sorry I don't remember the names of the bars I went to. Just know there are more opportunities for gay nightlife there than the other two cities. Have fun.