The Hyers-Ulam stability, the Hyers-Ulam-Rassias stability, and also the stability in the spirit of Gavruţa for each of the following quadratic functional equations f ( x + y ) + f ( x − y ) = 2 f ( x ) + 2 f ( y ) , f ( x + y + z ) + f ( x − y ) + f ( y − z ) + f ( z − x ) = 3 f ( x ) + 3 f ( y ) + 3 f ( z ) , f ( x + y + z ) + f ( x ) + f ( y ) + f ( z ) = f ( x + y ) + f ( y + z ) + f ( z + x ) are investigated.