ROT13 is normally a fairly weak encryption method. While it is not as susceptible to meet in the middle attacks as DES, it makes sense to use the same basic technique to strengthen it.
Triple DES is 3 rounds of DES (with different keys). Triple ROT13 is 3 rounds of ROT13; the same key can be used for all 3 rounds, of course. The encryption strength is effectively 3 times that of normal ROT13, unlike triple DES, which is only twice as strong as DES.
To decode triple ROT13, run the ciphertext 3 times through a ROT13 decoder. Or use your magic decoder ring.
E2 now does ROT13 encoding and decoding for you! Just visit the E2 ROT13 Encoder (really, it's also a decoder), and click your heels 3 times to check it out!