I never heard of this before now but activity is a component for "regular."
They can get to trust level 3 by...
This is time and activity based, unlike the previous levels. In the last 100 days...
must have visited at least 50% of days
must have replied to at least 10 different topics
of topics created in the last 100 days, must have viewed 25% (capped at 500)
of posts created in the last 100 days, must have read 25% (capped at 20k)
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)
must not have been suspended
* These likes must be across a minimum number of different users (1/5 the number), across a minimum number of different days (1/4 the number). Likes cannot be from PMs.
All of these must be true.
Unlike other trust levels, you can lose trust level 3 status. If you dip below these requirements, whether from flagging or the passage of time, you will be demoted to Member. However, in order to avoid constant promotion/demotion situations, there is a 2-week grace period immediately after gaining Trust Level 3 during which you will not be demoted.