#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

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

#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

#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

#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

^{_{This is slightly outdated (by 1 byte), but the algorithm is the same.}}

                      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
             rG1                 uppercase alphabet
           i       2            parse list of bools as binary
  @                            get correct item of
   s+rG1U6                       uppercase alphabet + nums 0-5
s                            concatenate and print

#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