1/(1*3) + 1/(3*6) + 1/(6*9) + 1/(9*12) + …+1/(3n*(3n+3))

=1/3  +  1/3*(1/3-1/6+1/6-1/9 + ......-1/3n+1/3n-1/(3n+3)

=1/3  +  1/3*(1/3-1/(3n+3))

=1/3*(4/3-1/(3n+3))