Works on OS X, Linux, anywhere with OpenSSL installed: To encrypt a file:
openssl des3 -salt -in infile.txt -out encryptedfile.txt
To decrypt the file:
openssl des3 -d -salt -in encryptedfile.txt -out normalfile.txt
Do not specify the same file as input and output on encryption.. I have noticed weird effects on OS X (it eats the file). Remove the -in * stuff if you want to pipe data into it (e.g. a tarred folder). Omit the -out * stuff if you want it to pipe data out on STDOUT.