Who says they have to be flat? I assume the approach legs are existing and are your constraint, correct? Each leg's centerline defines the first pass grade on the roundabouts profile, you have 3 VPI's to meet. You'll likely have to back up on each leg a nominal distance to adjust their pavement cross slope to meet that of the proposed roundabout's. The cross slope adjustment rate may not be specifically defined, if so review similar horizontal curve tables in AASHTO's Green book & your local DOT.
Judgment will come to bear on how much you can accommodate this depending on surrounding impacts, ... I'm always inclined to tackle final intersection grades by hand, using spot elevations to the nearest 0.1 of foot for my first pass and constantly checking cross slope in multiple directions to insure positive drainage. I usually do this on a large scale paper plot (10 or 20 scale) of the pavement jointing plan for PCC or lane lines for HMA.
