Two thin biconvex lenses have focal lengths $f_1$ and $f_2$. A third thin biconcave lens has focal length of $f_3.$ If the two biconvex lenses are in contact, the total power of the lenses is $P_1$. If the first convex lens is in contact with the third lens, the total power is $P_2$. If the second lens is in contact with the third lens, the total power is $P_3$ then