Perhaps, you unconsciously mistyped the x coordinate of third point. I use 3800 instead of 2100, but use what the actual value is. Also, I use random Z data here for test purposes.
x = 1000:4500;
z = 11+randn(1,length(x)); % use the data that you got
plot(x,z,'om'), hold on
x1 = 1000:2100;
y1 = 8+(8-13.2)/(1000-2100)*(x1-1000); % first boundary, straight line equation
z1 = z(1:length(y1)); % z values in the given range x1
z1(z1 >= y1) = NaN; % discard z values that are above the boundary
plot(x1,y1,'r'), plot(x1,z1,'o','MarkerSize',12)
x2 = 2100:3800;
y2 = 13.2+(13.2-14.6)/(2100-3800)*(x2-2100); % second boundary
z2 = z(length(y1)+1:length(y1)+length(y2)); % z values in the given range x2
z2(z2 >= y2) = NaN; % discard z values that are above the boundary
plot(x2,y2,'r'), plot(x2,z2,'o','MarkerSize',12)
x3 = 3800:4500;
y3 = 14.6+(14.6-12)/(3800-4500)*(x3-3800); % third boundary
z3 = z(length(y1)+length(y2)-1:end); % remaining z values
z3(z3 >= y3) = NaN; % discard z values that are above the boundary
plot(x3,y3,'r'), plot(x3,z3,'o','MarkerSize',12)
