Hi Jan Ali,
It is my understanding that you want to add three signals that are defined only in particular intervels of time. Following code might help you.
t = (-1:0.01:1); % defining time axis
unitstep1=t>=0; % unitsetp1 takes 1 only when t is greater than equal to 0
unitstep2=t>=0.1; % unitsetp2 takes 1 only when t is greater than equal to 0.1
unitstep3=t>=0.18;% unitsetp3 takes 1 only when t is greater than equal to 0.18
unitstep4=t>=0.3; % unitsetp4 takes 1 only when t is greater than equal to 0.3
rect1=unitstep1-unitstep2; % rect1 takes 1 only when 0<=t<0.1
rect2=unitstep2-unitstep3; % rect2 takes 1 only when 0.1<=t<0.18
rect3=unitstep3-unitstep4; % rect2 takes 1 only when 0.18<=t<0.3
% taking three random sine siganls
Signal1=sin(2*pi*50*t);
Signal2=sin(2*pi*150*t);
Signal3=sin(2*pi*500*t);
% Composite signal
Sc= Signal1*rect1+Signal2*rect2+Signal3*rect3;
