They describe a leaderboard built from server members' shared gaming activity, while actual rollout remains unconfirmed.
When It Appears
“Top three” can appear on a server view that ranks shared gaming activity and shows a timed competition when Discord displays the section heading highlighting the top three leaderboard entries.
What It Means
When “Top three” is shown, the current step is presenting the section heading highlighting the top three leaderboard entries.
A user who saw “Top three” in Discord is likely searching to confirm that it means the section heading highlighting the top three leaderboard entries, where it appears, and which state it describes.