Convert Salesforce 15-character ID to 18-character
Pyth, 23 22 bytes
1 byte saved by FryAmTheEggman.
sm@s+JrG1U6i}RJ_d2c3pz

Try it online. Test suite.
This might be the first time I've used the print instruction in golfing.
Explanation
     JrG1                   save uppercase alphabet in J
                     z      input string
                    p       print it without newline
                  c3        split into 3 parts
 m              d           for each part:
               _              reverse
            }R                map characters to being in
              J                 uppercase alphabet (saved in J)
           i     2            parse list of bools as binary
  @                           get correct item of
     J                          uppercase alphabet (saved in J)
   s+    U6                     add nums 0-5 to it
s                           concatenate and print