Good post here about it:
https://us.forums.blizzard.com/en/wow/t/finding-out-your-trust-level/20089
In particular:
TRUST LEVEL 3
Get to trust level 3 by…
- 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 over the last 100 days.
- Must not have received more than 5 spam or offensive flags (with unique posts and unique users for each, confirmed by a moderator)
- Must not have been suspended (including forum suspensions or in-game account actions of any Blizzard game)