Hi mate,
This seems to be a very very common issue on these mice. My MX anywhere 3 is doing the same thing with the right click after 4 years where it feels mushy, like something's dislodged.
Because logitech & iFixit have partnered, there are guides on this very website on how to disassemble the mx master 3s, and given the one you want isn't there you can always reference the master 3 guide, since they are very similar.
As for where to find the parts, Logitech isn't selling them here as they likely require resoldering. What you could do however is to open up the mouse, check what the part number is and given you are able to solder, order one yourself & do the repair.
If you don't want to do the repair yourself, there's a chance your local repair shop might do the repair for you since it's a small soldering job. Otherwise i don't reckon Logitech are going to do replacements on that mouse.
Do note, since this is less than a year old, you should be able to claim with logitech given the 1 yr warranty.