Matlab results not matching theory

clear all;
clc;
Fs = 1000;
Ts = 1/Fs;

t=-1:Ts:1;

%u(t-0.5)
step = zeros(size(t));
step(t>=0.5) = -1;

figure
grid
plot(t,step)

%u(t+0.5)
step2 = zeros(size(t));
step2(t>=-0.5)= 1;

figure
grid
plot(t,step2)

xt=step2+step;
figure
plot(t,xt)

y = conv(xt,xt);

t3 = -2 :Ts:2;

figure
plot(t3,y)

z = conv (y,xt );

t4= -3:Ts:3;

figure
plot(t4,z)

q = conv(z,xt)
t5 = -4:Ts:4

figure
plot(t5,q)

nfft=length(y)*32;
Y = fft(y,nfft);
Y = fftshift(Y);

F=-Fs/2:Fs/nfft:Fs/2-Fs/nfft;

figure
plot(F,abs(Y))

nfft=length(z)*32;
Z=fft(z,nfft);
Z=fftshift(Z);

F=-Fs/2:Fs/nfft:Fs/2-Fs/nfft;

figure
plot(F,abs(Z))

nfft=length(q)*32;
Q=fft(q,nfft);
Q=fftshift(Q);

F=-Fs/2:Fs/nfft:Fs/2-Fs/nfft;

figure
plot(F,abs(Q))

So I have this code, but for the 2nd convolution and 3rd one ( z and q ) the theory shows different results . Is there something wrong in my code or should I recheck the theory and redo it ?

[Image Analyst]

What are you trying to show? Please put comments in. Why are you adding negative and positive values to the ends of the arrays?

[Bruce Pollom]

What I have to do is convolute the base signal ( x(t) = u(t+0.5) - u(t-0.5 ) 3 times ) in time and frequency.

[Image Analyst]

What do you expect the base signal to look like? A square pulse? A negative pulse on the left and positive pulse on the right? I still don't know why you're tacking numbers onto the ends!

[Bruce Pollom]

The base signal looks good, and the first convolution aswell. My problem is at the 2nd convolution where I get a concave portion followed by a convex one in the middle and a concave one on the right but I get the opposite in theory. And what do you mean by tacking numbers onto the ends ?