                      B=rep(letters[1:3],3),
                      y=rnorm(9),
                      x=rnorm(9),
                      z=rnorm(9))

aggregate(da[,-c(1:2)],
               list(A=da[['A']],
                    B=da[['B']]),
               function(x) c('Média'=mean(x),
                                  'Desvio'=sd(x),
                                  'CV'=sd(x)/mean(x)*100))

(S,f,P)

Allaman