3DES Block Size
While the effective key length for 3DES is clearly much larger than
for DES, the block size is, unfortunately, still only 64 bits. For
CBC mode (the most commonly deployed mode in Internet security
protocols), this means that, due to the birthday paradox, information
about the plaintext begins to leak after around 2^32 blocks have been
encrypted. For this reason, 3DES may not be the best choice for
high-throughput links, or other high-density encryption applications.
At minimum, care should be taken to refresh keys frequently enough to
minimize ciphertext collisions in such scenarios.