Hi, I haven't stayed in any of them so can only comment on location.
The Kristal Palace and Garda hotels are quite far from the centre of Riva and the boat dock (20 minutes by the main road, 30 minutes by the lake path), but they are close to Porto San Nicolò, the lakeside path to Torbole and the start of the path up to Monte Brione (great views over Riva and Torbole). There are several restaurants and gelaterie nearby.
The Luise is closer to the centre of Riva than the other two but I don't much like the location - to the front there is probably a good view of the lake from the higher rooms, but immediately in front there's an empty lot which is currently used as a car park of sorts. To the rear there are views to the mountains but also the nearby paper factory.
You can see the locations here:
You can read the reviews here: