We bought a MacBook Pro 2017 that shipped with Sierra 10.12. Two month later bought two more MacBook Pros 2017 that shipped with High Sierra. I have a bootable USB of Sierra. Will I be able to install it on the MacBooks that shipped with High Sierra?
IMHO I wouldn't do it. I'd leave HS installed and upgrade the one with Sierra... or wait for the next few updates of HS before upgrading the Sierra Macbook Pro.
You also want to be careful if the new Macbook Pros with HS are using APFS which is probably the case. I'm not sure how Sierra would handle the downgrade... Re-format using HFS plus or what? ...Others here could confirm how this would work for you. You're going to want to eventually upgrade to High Sierra anyway within the next two years for continued MacOS support.
Again unless there is some critical need to have Sierra installed I'd leave it as is and maybe wait until the HS updates mature a little more before updating the MBP with Sierra to HS. Always make sure you have solid backups before doing anything.
