In the Books of A

B A/c Dr. 100
To, Cash     100

(C is just a third party, Receiving the money on hehalf of 
B. As in the books of A there is no out standing against C 
but it is against B)

In the Books of C

Cash A/c Dr. 100
To, B A/c 
(In ths books of C it must be receivable from B, so B must 
have said to A, that the money which you owe to me please 
pay directly to C

In the books of B

C A/c Dr. 100
To  A A/c    100
(paid by A to C)

C Account dr 100
   TO B Account100

although C is also involved in this transactions,however 
it`s not going to make any difference in this particular 
transaction.refrence of C can be given in narrations while 
passing such entry.
         B          Dr.  100
           To Cash          100
(being rs.100 paid to C on behalf of B)

another way of dealing this transaction is as following..

(1)         C          Dr.   100
              To  cash          100

(2)         B          Dr.   100
              To    C           100

the ultimate effect is
            B          Dr.   100
              To    cash       100

B a/c dr       100 
To cash a/c            100
(Being paid to C on behalf of B)
As per the instructions we paid to c we need not debit C in 
our books of accounts

in "A" BOOKS

'B' A/C DR.
 TO CASH A/C
(AMOUNT PAID TO 'C' ON INSTRUCTION OF 'B')=SHORT NARATION