the log out/in method only works (if at all) here, after you have "finished" Nobles, but not entered Catacombs yet - in this case there might be a chance to still get the 2nd boss
(you will be able to access the area since the barricades are off after you survived this stage)
for the 3rd boss I don't think this would work: killing the final boss will bind you to this instance and you would need to stay outside for 30 minutes;
- if that method works here at all, as the final kill will normally clear the instance