what dose he discover about his room in stave 3?

His room has been completely transformed into bright and lively room filled with Christmas decorations and filled with food and treats

Check out this quote from stave 3,

"It was his own room. There was no doubt about that. But it had undergone a surprising transformation. The walls and ceiling were so hung with living green, that it looked a perfect grove; from every part of which, bright gleaming berries glistened. The crisp leaves of holly, mistletoe, and ivy reflected back the light, as if so many little mirrors had been scattered there; and such a mighty blaze went roaring up the chimney, as that dull petrifaction of a hearth had never known in Scrooge's time, or Marley's, or for many and many a winter season gone. Heaped up on the floor, to form a kind of throne, were turkeys, geese, game, poultry, brawn, great joints of meat, sucking-pigs, long wreaths of sausages, mince-pies, plum-puddings, barrels of oysters, red-hot chestnuts, cherry-cheeked apples, juicy oranges, luscious pears, immense twelfth-cakes, and seething bowls of punch, that made the chamber dim with their delicious steam. In easy state upon this couch, there sat a jolly Giant, glorious to see:, who bore a glowing torch, in shape not unlike Plenty's horn, and held it up, high up, to shed its light on Scrooge, as he came peeping round the door."