Undo tablespace is not reclaimed

[jmodic]

one suggestion- reduce undo_retention for that particular session only...if you still wish to keep it system wide at this 3 hrs..
eg
alter session set undo_retention =1800 (for 30 minutes)

You can't set UNDO_RETENTION on the per-session basis! So, "alter session..." is a no-go in this case.