FontProperties are shared by all three titles of an Axes object

[fcolas]

When defining center, left and right title, the font size and weight will be set for all of them.
For example:

import matplotlib.pyplot as plt
ax1 = plt.subplot(121)
ax1.set_title('center', loc='center', fontsize=20, fontweight=700)
ax1.set_title('left', loc='left', fontsize=10, fontweight=400)
ax2 = plt.subplot(122)
ax2.set_title('left', loc='left', fontsize=10, fontweight=400)
ax2.set_title('center', loc='center', fontsize=20, fontweight=700)
plt.show()

Left plot will have both titles small whereas right one will have both of them large and bold.