The best replacement for phpbb3 is smf.

The best alternatives to phpbb3 are rated as following:
★★★★★  smf
★★★★  phpbb2
★★  vbulletin
★★  invision
★  ipb
★  mybb
★  vbulletin 4
  kunena
  vanilla

Your alternative for phpbb3 is not listed? Suggest!
Speaking about phpbb3, one probably shouldn't forget about cms.

Other search terms related to phpbb3 and their ratings are:
★★★★★ cms
You have in mind a companion for phpbb3 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of phpbb3 in all years. Click on graph for more.
phpbb3
smf
phpbb2