You are confusing functions. conv is used to obtain the convolution, subplot is used to divide a figure and then plot inside each subregions. For example
time=0:0.1:10;
x=cos(time);
y=sin(2*time);
z=conv(x,y);
figure
subplot(3,1,1)
plot(x)
subplot(3,1,2)
plot(y)
subplot(3,1,3)
plot(z)
Hope this solves your question.
