Congratulations on your first trip to Europe!
I went to Europe every year before the pandemic, and with one very small case, and last time only a carryon.
You will be amazed at how many outfits you can make out of a very small amount of pieces.
I have to say that I'm slightly in horrified awe of anyone who plans to travel with three suitcases.......are you moving there?
The first thing I would suggest to you is to watch as many packing videos as you can, and take notes.
Read about packing lighter, and make lists.
As far as trains go, find some videos on youtube about using Italian trains, and you will see that they are easy to use.....but...only if you have less rather than more luggage.
Listen to everyone here.....we have all started out taking far too much stuff on our travels, and learned from experience to really pare down.
You can still look very smart with way less stuff!
I will never forget the man sitting on a giant suitcase in the train station in Florence.
He had come from Australia, and was traveling by train every couple of days with this giant case.
He was sweating, grumpy and hot; and told us he was going home early as it was just too difficult to get around with this suitcase.
I don't know where you live, but Italy in summer is hot and very humid.