Three nights away does not need a suitcase you have to check. A carry-on works if you decide what the trip actually is before you start folding. A city, a walk, one dinner you care about, and weather that might change. That is the whole brief.
Clothes
Take what you will wear twice. One pair of shoes you can walk in, and the shoes you travel in if they are the same pair, even better. A layer for the evening if the day was warm and the night is not. Underwear and socks for each day plus one spare, because a wet morning is how a small bag fails. If the trip includes a flight and a dinner, choose the darker, simpler clothes. They come out of a bag looking like you meant them.
The small things that save the bag
A toothbrush, the medicine you actually take, a charger, and a plug that fits where you are going. Leave the second bottle of anything you can buy if you run out. Liquids are what make a carry-on feel illegal at security. Decant what you need into small bottles and leave the rest at home. A paperback or a downloaded book is lighter than a second pair of shoes you will not wear.
How to know you are done
Close the bag and pick it up. If you resent it in the hallway, you will resent it on a cobbled street. Take one thing out. The trip is three nights. Nobody is judging the outfit you repeat, and the bag you can lift with one hand is the one you will actually enjoy having with you.