They're virtually the same - the 3s is an upgrade over the 3 which is different from the 2019
MX Master 3 that it’s replacing: it’s got a higher resolution 8,000 DPI sensor (up from 4,000 DPI last time around) and a silent click mouse click (which is still tactile) that Logitech claims is 90 percent quieter, and it comes with an updated USB receiver as well as some minor updates to the scroll wheel and interior.
I personally LOVE the silent (yet still tactile) clicking - I can work away in a quiet house in the early hours of the morning without anybody knowing anything different, or in a cafe without annoying their person next to me, or in a silent area without annoying myself LOL. I used a mx anywhere two before this and while not a loud mouse, I'm grateful for the silent clicking on the 3s and have 3 of them - one at home, one at work, and one for travel.
In saying that, the 3 can be found on sale - but, ahead of Black Friday I'm sure you'll be able to grab a great deal on the 3s. I wouldn't upgrade from a 3 to a 3s based on cost, but if buying one from scratch I'd go with 3s if the silent clicking intrigues you
Happy shopping