To maintain trust level 3 you simply need to make sure that at any given time you fulfill the conditions of trust level 3.
- Must have visited at least 50% of last 100 days
- Must have replied to at least 10 different topics
- Of topics created in the last 100 days, must have viewed 500 topics
- Of posts created in the last 100 days, must have read 20,000 posts (both starting posts and replies)
- Must have received 20 likes, and given 30 likes.
- Must not have received more than 5 spam or offensive flags (with unique posts and unique users for each, confirmed by a moderator) (If this happens it simply resets your stats back to zero and you have to rebuild)
- Must not have been suspended (including forum suspensions or in-game account actions of any Blizzard game) (If this happens it simply resets your stats back to zero and you have to rebuild)
