Currently we at BlenderArtists.org have 35 TL3 users, it’s been as high as ~50 but it fluctuates quite a bit.
TL3 users get a lot of perks on our site- notably, they get to vote on artwork to be “featured” and receive extended visibility on the top row of the site (this is partially an art-sharing forum). Voting seems to be highly motivating; most users that become TL3 stay TL3, even if they drop off for a period they almost always come back.