Eliminating a 3 way switch in a 3 gang outlet
Yes, if the switches are all on the same circuit (breaker), you tie all the neutrals together.
Yes, you can have dumb &amp; smart switches in the same box. And receptacles. And all kinds of other things.
As Harper noted, you must have switches for stairway lighting at top &amp; bottom. So if you want to put in a smart switch for one end, put in a smart switch remote for the other end.