This is one of those things that people say, but how do we know it's true? For example, how many teams have had three All-conference level forwards and/or centers on the same team in the past 10 years? How did those teams do in their conference and/or NCAA tournament? I can't imagine the sample size is too large, especially if you discount teams that have three great forwards/centers and also have good guards. And if we haven't seen many or any teams like that, how do you know a team with that construction can't win a championship?
Have you seen enough of Keels to know he's capable of dribbling and passing out of pressure? Have you seen enough of him to know he can defend and pressure other teams' guards? Have you seen enough of AJ Griffin to know he's not capable of those things, despite being a forward?
Personally, I think good players and coaches win championships. Assuming the team's pieces fit, it doesn't matter how many "true" guards you have.