You need to plot both the x and y vectors in your plot statements.
This works:
tt = -0.00025:0.00001:0.00025;
x1 = 20*cos(2*pi*(4000)*(tt -37.2/10));
tt = -0.00025:0.00001:0.00025;
x2 = 24*cos(2*pi*(4000)*(tt +41.3/10));
x3= x1+x2;
subplot(3,1,1)
plot(tt,x1)
title('X1')
xlabel('Time(sec)')
axis([min(tt) max(tt) ylim])
subplot(3,1,2)
plot(tt,x2)
title('X2')
xlabel('Time(sec)')
axis([min(tt) max(tt) ylim])
subplot(3,1,3)
plot(tt,x3)
title('X3')
xlabel('Time(sec)')
axis([min(tt) max(tt) ylim])
The axis function limits the axes to what you specify. If you want them to go from [-3,3] instead, simply remove the axis calls.
