Which railway number has 3 quantity?
CREATE TABLE "narrow_gauge_locomotives" (
  "class" text,
  "railway_number_s" text,
  "drg_number_s" text,
  "quantity" real,
  "year_s_of_manufacture" text,
  "axle_arrangement_uic_bauart" text
);
SELECT "railway_number_s" FROM "narrow_gauge_locomotives" WHERE "quantity"=3;