I see the following on my machine:
kevin@ubuntu:~
$ locate libsmime3.so
/usr/lib/firefox/libsmime3.so
/usr/lib/thunderbird/libsmime3.so
/usr/lib/x86_64-linux-gnu/libsmime3.so
kevin@ubuntu:~
$ dpkg -S /usr/lib/x86_64-linux-gnu/libsmime3.so
libnss3:amd64: /usr/lib/x86_64-linux-gnu/libsmime3.so
I think this implies that
sudo apt install libnss3
should provide the needed library.