I have 2, nine-inch spring form pans. Could I use these in place of the three 8 inch pans? If so, how should I adapt?

It will be very slightly deeper with the 9" pans, so will probably take a couple of minutes longer. The recipe gives a fairly wide time window anyway- start checking at 25 min. and you should be fine. You may also get less browning on top because of the depthe of the pans, but it shouldn't matter much.