Will the RPi3B+ be only used as a NAS unit? So most likely a headless setup.
Make sure your power supply unit conforms with the RPF recommended rating of 5V 2.5A . Below that you'll likely encounter problems later if not sooner.
To prevent "accidental shorts" to the RPi board during your testing, better provide a good case for the RPi. Start with some non-metallic cases. Upgrade to better ones later.
Next you need to familiarize yourself with the Operating System to use for your NAS System.
Immediate pick will be Raspbian Stretch Lite.
https://www.raspberrypi.org/downloads/raspbian/
Most of the members here will recommend you flash Raspbian to microSD card using Etcher.
https://etcher.io/
There are procedures here in the forum on how to properly boot the headless RPi.
Once you've past the initial setup, update and upgrade next to do is install file sharing service. Samba is the easiest one to setup.
Again there are several tutorials available on how to setup samba on an RPi. Take time to read those carefully before proceeding.
Once you've setup samba and happy with the results, make sure you make an image backup of the microSD card, so you wont have to do everything from scratch when some trouble arise later.
For the USB thumbdrive and/or USB HDD, it is recommended that it be formatted to EXT4, If in case you may need to plug the USB HDD or USB thumbdrive to a windows pc, better format it to NTFS format, and install ntfs-3g to raspbian so that you can have good access to the NTFS formatted volumes on the USB HDD/USB thumbdrive.
If you do encounter some errors, problems, troubles during your testing and or final setup, there are a
band of few good folks here that will offer assistance in finding solutions....
Good luck with your build!