Drupal Commerce Authorize.net gateway and POODLE
The commerce_authnet module is using the PHP curl() functions to communicate with Authorize.net, and it does not explicitly requests the SSLv3 protocol in the curl options (see file commerce_authnet.module, lines 1163 to 1172).

curl will therefore rely on the HTTPS handshake to determine the protocols and ciphers, and since the Authorize.net will not advertise SSLv3, another protocol will be selected during the handshake (most certainly TLS v1.2).

So you have nothing to change, the transition will be handled automatically by the HTTPS handshake. I would only advise you to upgrade your openssl library to the latest version.