The Citaro doesn't have a flat platform.

Look at the seats at the rear, they go upwards (ie the further
you go, the seats are higher, and the fact that there is a raised
platform on the rear left side (when looking at the front) makes it
extremely difficult to configure a RHD Citaro with a 3rd door, even
in LE configuration.

Source : Google
I'm not really referring to that though. Somehow, I remembered
that I once suggested for this configuration for the MAN A22. The
comments I gotten was "steep slope", "too high", "uneven" blah
blah..
So I assumed that Citaro have better platform(almost flat),
which is ideal for such idea for a tri-door buses in SG.