Each time through the loop, Th3 is being overwritten with a new value. You need to assign each new value to the next index in an array.
theta_range = 0:360; % create the range of angles
Th3 = zeros(2, numel(theta_range)); % preallocate array
Th4 = zeros(2, numel(theta_range)); % preallocate array
for theta_index = 1:numel(theta_range)
% step through angles (this helps avoid zero indexing and lets you have a non-integer range)
Th2 = theta_range(theta_index);
d = 3.5;
a = 1;
b = 2;
c = 4;
Th1= 0;
Z = d*exp(1i*deg2rad(Th1)) - a*exp(1i*deg2rad(Th2));
Zc = conj(Z);
Ka = c*Zc;
Kb = Z.*Zc + c^2 - b^2;
Kc = c*Z;
T = roots([ Ka Kb Kc]);
S = (c*T+Z)/b;
Th3(:, theta_index) = rad2deg(angle(S)); % this function gives you a 2x1 result, so put it in both rows
Th4(:, theta_index) = rad2deg(angle(T));
P = a*exp(1i* deg2rad(Th1))+ 6*exp(1i* deg2rad(Th3+20));
end
subplot(2,1,1)
plot(theta_range, Th3(1,:));
xlabel('Theta')
ylabel('Positive Theta 3')
subplot(2,1,2)
plot(theta_range, Th3(2,:));
xlabel('Theta')
ylabel('Negative Theta 3')
