Pros: 1.) The fact that the scroll wheel is all metal or at least gets rid of the stepped plastic insert from the 2S. Cleaning that part of the mouse was a minor quibble on the 2S nonetheless. Either way, the all metal scroll wheel feels a lot better to use now that it's all metal and the actual groves in the wheel feel much better to touch.
2.) The silent clicks on the right and left clicker are a positive in my mind.
3.) USB-C is a plus. Charging is faster although I didn't bother to time the exact difference between the 3S and my 2S. Future-proofing is a good thing given that the future is USB-C
4.) Bigger horizontal scroll wheel with a caveat though (see con#1). No plastic grippy insert for the the horizonal scroll wheel on the 3S which is a pro or a con depending on how you use the horizonal scroll wheel. I didn't mind it either way on the 2S and not having it on the 3S is the same for me.
Cons: 1.) Logitech moved the dedicated side buttons lower and placed the horizontal scroll wheel above the dedicated side buttons. That means my thumb is constantly touching the lower of the two buttons on the left side which is not my preference. Compared to the MX Master 2S, the 3S two side buttons are too close to my thumb now which means I accidentally click the (default) forward button on the 3S more than I ever have when I used the MX Master 2S.
2.) The rubber or soft-touch plastic that surrounds where your palm goes is way too slippery for my taste. The MS Master 2S had a grippy soft outer rubber layer for the thumb and the palm which made it much easier to hold and grip. Unfortunately the MX Master 3S is very slippery and harder to grip. The MX Master 3S is definitely much easier to clean but holding the mouse requires me to use my pinky and right ring finger more often. Compared to when I used my Master MX 3S, I could easily and swiftly wander with the 3S using only my pinky and thumb to hold the right and left sides respectively.
3.) Activation/press area of hidden thumb button is much smaller on the 3S compared to the larger area on the 2S. There's a little grove/pit that you press down on using your thumb with the 3S and it's much smaller than I would like. Not only is it smaller on the 3S, the position of the area/grove is too far behind where my thumb rests. I have to scrunch up my thumb to reach back on the mouse to click it which is annoying. The 2S had a much better placement and the entire area your thumb rest on was clickable. Yes, the 2S had that hard plastic edge that made it harder to clean and allowed things to get into which made it even harder to clean but since I always have clean hands it was never an issue for me with I used the 2S hidden thumb button.
4.) The 3S has a smaller palm area. Depending on your preference, this is a con or a pro or both. For me, this is a con. I have small hands so I liked how much more bulkier the MX Master 2S was. The bulkier 2S was easier for me to hold because I didn't have to scrunch up my palm in order to make more contact with the palm rest. This also means less area for your palm to rest on which I do. If you don't rest your palm on the mouse as much, this won't be a con for you.
Not a pro but not a con either: 1.) No storage area for the Bolt receiver on the 3S anywhere just like the 2S unfortunately. I don't travel a lot so it's not a problem at the moment but I would have still liked to see a tiny compartment to place the Bolt receiver into when not in use. One of my old Logitech wireless mouse had a removable battery compartment which allowed you to place the receiver into when it wasn't in use and that was great. Now I can and have lost a few receivers because there's no place to put those tiny Unifying receivers.
2.) No visible charge level indicator. Yes there is a charging indicator but it doesn't tell you the battery level/percentage without having to check the software. The green light is simply on of off. I don't know what it looks like when there is a low battery (like if it turns red or anything) but I'd rather know the actual percentage just from glancing at the mouse instead of having to open up the software and constantly check. For example, three tiny lights to tell me if the battery is high, medium, or low. Notifying me only when it IS low on battery is literally waiting until the last minute to let me know before I have to charge it which isn't a plus exactly.
3.) Ambidextrous it is not. I'm not left handed or ambidextrous but I really would have liked to have seen an ambidextrous version of the MX Master series from Logitech.
4.) You can use it even when it's charging (looking at you Apple and Sony). I hate not being able to use my devices when they're charging especially because I don't like to plus in my devices to let them charge (and then overcharge) overnight. I'm glad Logitech didn't try to reinvent the wheel just to look sleeker and dumber like Apple's pathetic (not so) Magic mouse.
5.) No USB-C receiver. I switch between a desktop using Windows and a MacBook Air. The Windows desktop uses the Bolt receiver and the MacBook Air uses Bluetooth. The Bluetooth use for this 3S mouse is a little laggy on my MacBook Air but I don't mind it much since I code as a hobby. My Windows desktop is my main computer. I don't have a Bolt receiver for my MacBook Air yet but that's because I'm a.) waiting to see if Logitech gives us a Bolt receiver with USB-C and b.) I don't have a docking station/hub yet to use with my MacBook Air anyway (and the MacBook Air I have only has USB-C ports). The reason why this isn't a con or a pro is because my Windows desktop doesn't have any USB-C ports to begin with because of COVID and the chip shortage with those eye-watering prices for computer components. Otherwise I would have built a new desktop computer by now.
I don't know if Logitech even reads these reviews but I really hope the next iteration isn't worse. It's becoming increasingly more difficult to find tech you like to use only to not be able to find a replacement down the road because the series was discontinued or has been "upgraded" by a manufacturer. Either way though, I'm glad we have the 3S. I can certainly imagine a much worse product/world if we didn't have the 3S. Things could have been worse with the 3S but thankfully they're not world-changing bad at least for me.