Interesting situation. I think you might be able to get there from here using the new delay feature of dialer-watch. Here's a document on dialer-watch:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t8/ftdialwl.htm
Configure the backup links to come up as you normally would, with floating statics or whatever's to your taste. Configure a dialer-watch group watching some critical route, with a delay of say, 60 or 120 seconds and apply that to your tertiary backup site's dialer. In this manner, if you lose your primary connection AND your secondary connection refuses to come up for a couple of minutes, dialer-watch releases your tertiary dialer and it can go to work.
Hope this makes sense, post if you have questions.
