From your description of the location—-a San Marco palazzo on the Grand Canal, near either the San Tomà or S. Samuele vaporetto stop, I believe it could be Granda Sweet Suites. This is right at S. Samuele, which is in sestriere San Marco (San Tomà is not). This is guesthouse, not a 3-star hotel. Note that 3-star hotels are required to have staffed reception (with staff in uniforms) at least 16 hours a day.
https://touritalynow.com/italys-hotel-star-ratings/
So perhaps you confused the 3.5 “bubbles” rating by guests on Tripadvisor with the hotel star system? The place gets a number of miserable reviews on Tripadvisor. Filthy, moldy showers, no room cleaning, doors won’t close, etc.
https://www.tripadvisor.com/Hotel_Review-g187870-d16745166-Reviews-Granda_Sweet_Suites-Venice_Veneto.html
And on booking.com, a number of reviews (which are genuine reviews by people who actually stayed there) mention the difficulty of checking in and the absence of staff.
https://www.booking.com/hotel/it/granda-sweet-suites.html?aid=311088&label=granda-sweet-suites-
My suggestion about Tripadvisor was to post a question in the forum about whom to contact to report the perceived safety violations, not to write a review. I do not see your question there yet. My suggestion is based on my personal experience with a similar matter that actually got results:
We were overcharged by a restaurant near Piazza Navona—-the waiter refused to bring us the proper change for our 50-euro note, claiming the extra was “tax” (which does not exist in Rome), and refused to provide a receipt as required by law. When I got back to the hotel and read the reviews, I saw the same thing had happened to many others. The place “La Fraschetta” at Via Agnone del Angelo 21, had the lowest possible rating on Tripadvisor (one “bubble”). (Please do not confuse this place with other Rome restaurants named La Fraschetta.
I wrote of our experience in the Tripadvisor forum and one person posted the link to file a complaint with the Guardia di Finanzia. I contacted them, with a narrative of our experience and a link to the Tripadvisor reviews. They responded with a request that we fill out an official complaint form and provide a sworn affidavit, which we did.
I heard nothing more from the tax authorities, but within 5 months the place closed. This was 8 years ago, so unfortunately you will not be able to find those old reviews.
I kept a lookout on the address and within a year a new restaurant, named “Antica Locanda” or something like that, opened at that location. Before long, according to reviews on Tripadvisor, they were up to their old tricks. That place also closed down, either during or right before the pandemic. Last time I checked, the place that has re-opened at that address was doing a legitimate business (but I haven’t looked lately).
My point is that it is worth finding the right agency for filing a complaint, and someone in the Tripadvisor forum, or maybe one of the Italian forum members here, might be able to help you with that.