>>106242969
>NAS
Easy. Not sure about how performant Pi3 is but still.
>wireless access point
Yes assumed you got an interface that's capable. And even if your random interface can establish an access point, beware it may not be able to function on the same wireless standard as it can on client mode.
I for example have two different Wi-Fi adapters - Realtek and Intel - both are 802.11ac - or "Wi-Fi 5" - but they can only do 802.11n ("Wi-Fi 4") access points. Realtek can do 5GHz and Intel not even that. Intel I got with my motherboard as it's integrated and the Realtek one I specifically bought for an access point use and kinda got disappointed, AGAIN.
Extra hard mode: do mesh adapters even exist?

>wireless access point to serve a file storage interface, without itself having internet access
Of course. Sounds like you are new to networking, assuming that a link would know anything about IP-addresses in itself.
Simplest possible solution would be to give the wireless interface an IP from an arbitrary local network space.
OR you could turn your Pi into something that bridges between wired and wireless: bridge those two adapters together and give the bridge interface an IP. (or don't give an IP if you wish it didn't speak IP but I guess you want NAS over IP, idk)