To equate the colors between the histogram and line pairs, access the object handles from the plot output and then set the SeriesIndex property to the same value. See a note at the bottom of this solution to control the color.
To show only the histogram objects in the legend, specify the histogram handles in the legend command. I also defined the legend strings using the DisplayName properties. Note that the fitdist plot will return a 1x2 vector of handles where the first handle is the line object and the second is the histogram.
x = (1:1:100)';
y = (101:1:200)';
z = (201:1:300)';
p1 = fitdist(x,'Normal');
p2 = fitdist(y,'Normal');
p3 = fitdist(z,'Normal');
h1 = plot(p1);
set(h1,'SeriesIndex',1)
h1(2).DisplayName = '5%';
hold on
h2 = plot(p2);
set(h2,'SeriesIndex',2)
h2(2).DisplayName = '10%';
h3 = plot(p3);
set(h3,'SeriesIndex',3)
h3(2).DisplayName = '15%';
legend([h1(2),h2(2),h3(2)]);
hold off
ax = gca();
ax.ColorOrder = [1 0 0; 0 0 1; 1 0 1];
