Hi
1) In general, if a development cum GPA, and even if a supplementary deed is unregistered, if a party who has been allocated flat in lieu of his share of property, alone can enter in to a sale deed and that sale deed is perfectly valid.
2) Under law, more specifically in your case, party(B) acquires title by a development agreement and supplementary deed will only convey the holders interest in a title, So in your case, though Only B has signed the Sale deed, your title is perfect given that B has acquired both title on the property (through DGPA) and interest (through unregistered supplementary deed) .
3) However as a matter of additional safety and in order to set any doubts you may have on your title, you can get the ratification deed executed by C and with the contents being
"And whereas now in the sale deed which was executed by B in the favor of Mr . Vamshi, and the other DGPA holder C who is now available in India hereby ratifies the said sale deed executed by B in favour of Mr.Vamshi , as to be binding on him(C) .
Hope this information is useful.