ok i know hes great at taking the ball forward... and its always great to have a centerback who can actually go forward and make something happen... but surely first and foremost ud be looking at his defending and whether he can consistently be solid in a defensive partnership...
from what iv seen of the lad he has many great attributes but hes also lacking in many others.. his positioning and aerial dominance and concentration arent the best around and having him in a 2 man central defensive pairing is always likely to cause u problems...
i think the comparison with ferdinand is a little way off ... ferdinand is a great defender... he can also play, but his main strengths lie in his defensive attributes... unfortunately its not the same for agger...
on paper playing him as part of a 3man back line sounds like a good idea... but to be perfectly clear on one thing.. i dont think that any top club should ever play a 3 man backline unless they have the right personnel to do so and im not talking about centerebacks only ... the only club that i can remember making it work in recent times was Roma but they had what it took and probably had the best wingbacks and midfielders to make it work at the time... cant see any other top club doing it atm..definately not liverpool with the players we have... national teams maybe but top clubs... dont think so..
Best solution i could think of was playing him as a leftback where his strengths can be frequently used for the benefit of the team while his failing wouldnt leave us as exposed.