sshadess I hope you don't mind, it was suggested in another thread that I post this here:
OK, in the spirit of another thread I made recently,
here's a map of some good spots for trios/quads in PP. Credit goes to 'Valacar's Maps'
http://www.valacar.com/rappelz/index.html for originally creating this map. All I did is circle the rooms and added the levels of the mobs therein.
These are MOSTLY private 3 mob rooms... (*private meaning most are at dead ends)
Although there are some public rooms/hallways shown, I didn't list
all of the public rooms (like HH, Gate 16, etc.) because some of those have risk of trains, people running through, etc. etc. The public rooms/hallways I did list are ones where it's rare to see people or trains.
Hopefully you find this of some use...

EDIT: Added credit/link to original map. Thanks Waif for the info.