Now that’s an interesting question… I’m sorry, but I think Windows 95 is the earliest version we ever supported. I’m just guessing, but I’d think you could find some old PGP version that might run on 3.11 and it might support 3-DES. Possibly also some early ports of unix crypt, although that might only support DES, not 3-DES (not sure).
Considering the alternatives, and assuming that you might get away with a simple command line utility, I’d try to setup a development environment for 3.11 and get some old C sources for crypt for example from minix or some early linux distro, and maybe swap the algorithm for AES – there are reference implementations in pure C that should compile. Turbo C was pretty good back in the day ;-)