Multiple instances of Bitcoin Core on the same computer cannot share the same blocks (or data directory) folder. They will run into simultaneous write issues and corrupt the data on the disk.
Wouldn't that not be a problem in this case because OP said 3 accounts on the
same computer, and to switch accounts, you need to log off, closing all running programs. Bitcoin Core would never be writing at the same time with other instances, so nothing would be corrupted.