"determined the rear catalytic converter was completely clogged."
The engine probably had raw fuel going into cats for it to completely clog. How long did you drive with with those cylinder misfiring? Any Flashing CEL?
Back pressure from the clogged cat probably caused the cylinder to fail.
^ This is what I was thinking. My guess is the extra heat/pressure from the clogged cat caused the head gasket to fail which caused lubrication or cooling issues for cyl 3. Lost a head gasket to an 85 Fiero back when I was in college in a 2.8 V6 because of a clogged cat.