If there does end up being a three way split of the delegates and the establishment decides that it has to be anyone but Trump then Cruz will be the nominee even if he finishes 3rd.
If Cruz finishes 3rd Trump will offer him the VP. Rubio will offer him the VP. Cruz then says to Rubio, either back me for the nomination or I support Trump and become his VP. Rubio then has to accept to stop Trump becoming the nominee.
So a Cruz, Rubio ticket is the best the establishment can hope for from a brokered convention.
In that case, the Establishment can disqualify Cruz on grounds of not being a natural born citizen and still have Rubio as the nominee.
