I don´t think you "should" nor need to take all three Elite units..
That said, I guess you can come up with some fluff for it and I used to have an army in 6th Ed using just two blocks of Longbeards as my main fighting units..
But i think it is too costly if you want to make something that is decent against all comers.. The Core are good at their job and cheap-ish. Warriors are good, for relative few points you get static combat resoloution. Just support them. Puta character in there and they will do the job.
I would say that you can make do without any of the Elite units.. I never used Ironbreakers in 6th Ed (which is what i played the most( - mainly because.. Ah the models were alright but i loved the Longbeards sooo much mroe and everyone spammed the Ironbreakers.. So I wanted to be different. And i LOVE those 6th Ed Longbeards.. Along with Brian Nelson´s Orc Warriors, my fav models.. Colin Dixon did them?
I never played before 6th Ed either so for me Longbeards always had great weapons.. So I would take those and use my fav models.. Make it work. If you get the charge they might be good, sometimes.. Or use the WS 5 and S4 hand weapon and shield..
I think it is excessive for more than one, possibly two Elite units in 2000p though. And you need some "beardlings" to lecture.. 😉