SQ20 / 29MAY / SINLAX is showing some available seats in C. None in the SQ12 or SQ30, though.
Also, sometimes there will be discrepancies between different GDS systems, so it could be a case of Galileo showing availability while Amadeus showing as sold out, for example. The only way to find out is to actually put forward a request for a seat through the GDS, and wait for SQ computer's reply (with a KK or HL)