So... I have removed the triangle on top of the "true" fins, to make them flat, a little smaller. Then I have glued the first parts and this is drying:

(the top Su-27 component will have the normal triangle on top of the fins, and its central "fin" will be at the rear, not to be used as support. The 2 other tailplanes will become foreplanes of the top component, while extra little parts could make a biplane canard foreplane on the lower Su-27. Crazy? yes, joyfully...)