What is the highest Quantity for PTRD Nos. 3, 15?
CREATE TABLE "locomotives_of_the_great_western_railway" (
  "manufacturer" text,
  "type" text,
  "quantity" real,
  "ptrd_nos" text,
  "gwr_nos" text
);
SELECT MAX("quantity") FROM "locomotives_of_the_great_western_railway" WHERE "ptrd_nos"='3, 15';