Piling a third task on an individual is always going to be a difficult call. Too many eggs in one basket.
I would imagine granting a second admin task, let alone the third, would suffer significant penalties. You are asking someone to run a whole section of your government, then run another, and another. That is never going to be easy. In RL I have previously handled the work load of what was five people at one point - what it meant was none of it gets done properly and I am well aware that there were tasks that needed doing that had not been done months after they should have been resolved and when this refers to in game running a country I would hate to see the effect on your economy, let alone your poor monks starving in their monastries.
Then is then also the fact that some people might object to your "favourite" being given yet another high profile position when he already has two..... Admittedly some of those might be people you want to recruit....
I would agree with Bjorn; what you are trying IS possible but it really is not very wise. And a poor example of rulership.
