https://en.m.wikipedia.org/wiki/Millennium_Prize_Problems
The Clay Mathematics Institute officially designated the title Millennium Problem for the seven unsolved mathematical problems, the Birch and Swinnerton-Dyer conjecture, Hodge conjecture, Navier–Stokes existence and smoothness, P versus NP problem, Riemann hypothesis, Yang–Mills existence and mass gap, and the Poincaré conjecture at the Millennium Meeting held on May 24, 2000. Thus, on the official website of the Clay Mathematics Institute, these seven problems are officially called the Millennium Problems.
The Poincare conjecture was solved now leaving six remaining problems
@strutheo You forgot Navier-Stokes as an option. Also, a suggestion: This might take a long time to resolve, so won't attract that much trading. Maybe something like "Which Millenium Problems will be among the last 3 unsolved ones?" would make sense?