central limit theorem , convolution

in probability course assignment about proving the central limit theorem "CLT" using matlab

http://en.wikipedia.org/wiki/Central_limit_theorem

given these two random variables

X=1/3(u(x)-u(x-3))
Y=1/4(u(x)-u(x-4)) , u(x) is a unit-step fun.

W=X+Y , the sum of the 2 random variables is the convolution of their density functions

and plot the results of summation of :

1. n=10 random variables 
2. n=20 random variables
3. n=50 random variables

please help me doing this assignment , professor gave us a hint that we can use convolution with for loop and plot the final function I can't figure out how I can do it thanks in advance I will appreciate any help with assignement :)