We were on the Eurodam last May to Alaska and we decided to over pack knowing that the weather could swing dramatically. Between coats, boots, backpacks, gowns, suits, and 4 outfit options per day, we ended up taking a very large (70 lbs filled) suitcase, a medium, and a carry-on size, along with a big duffle. The suitcases nested nicely into each other and there was no problem storing it under the bed. We used the duffle for dirty clothes which saved space in the hard cases for all of our souvenirs, although we did have many items shipped home so we didn't have our hands full in port.
Funny enough, the only day we wore our coats was on the bow at Hubbard Glacier. The wind was brutal. It was over 70 degrees in Juneau on our port day and only needed a light fleece on our glacier dog sled adventure.
If your small bag can nest into a medium or large that is what I would do. There is so much closet space to spread out clothes that you don't really need to worry about accessing your bags during the cruise but you can use that space for shoe storage, backpacks, etc instead of having the space taken by 3 small bags.