Cut a new piece of plasterboard about 1 inch larger all round than the hole. Punch small holes near each corner of this piece,say 3 inches in diagonally from each corner and thread string through. Wiggle the new piece through the hole,it should go through on a slant,apply some fairly quick drying adhesive around the border of the new piece then position it and pull it with the strings so it sticks in position to fill the hole. When dry, since it is a large area (3 x 3) perhaps you could stick a second piece roughly the size of the hole to get the level correct then finish off with filler in the remaining crack.
Mark Atherton. UK, March 2004