Third member bolts or trailing arm bolts - if there is anything to grab on to, the first thing that you should try is to heat the stub up with a propane torch a little. The heat will release the Locktite that might be holding things together. My guess is the bolts broke off inside the threaded section, right?
As others have suggested, you can try drilling down the center of the bolt, then inserting one of those tapered bolt extractors (available at the hardware store or finer tool shops) and try backing the remaining stub out. Usually it works.
Hope this helps!
