The only thing that keeps a serial interface from being put in the third slot is a little tab which could be easily bent or broken off.
The 4000 series is supposed to be WAN modules in the first two slots and a LAN module in the last, but in actuality it seems that you can do any combination.
If I bend the tabs, can I mix modules in a 2524 or 2525? Since I don't have an ISDN simulator, I really don't need an ISDN module, but a third serial could be useful.
This would not work, for instance, if part of the serial hardware was on the motherboard, or there was some other difference in the connectors on the slots. Or if the software enforced serial interfaces in the first two slots only.
I can suppose that Cisco might attempt to enforce slot usage rules to make customers buy more expensive routers rather than install strange combinations of cards in 2524s.