http://i45.tinypic.com/2wburdd.jpg
member 1 is exponential of lambda=1 X1~exp(3)
member 2 is U(2,4) X2~U(2,4)
member 3 works 3 hours and then dont work at all
X3=1 for t<3
X3=0 for t>3
i developed it at much as i could
i dont know how to continue from this point
to find the formula