Does anybody make 10cm pipe that is composed of 'cake slice' sections?

The reason I want to do this is because I have an existing pipe which is routed from my apartment to the roof. I now need to get three pipes up there to support the new combi boiler intake and outlet plus the bathroom extractor.
I am imagining a form of pipe of the same cylindrical pattern as the pre-existing pipe but with a Y-section form through the length of it so that it has three separate sections. i.e. so that a cross section would look like a pie chart of three pieces.
The reason I want this is for a bathroom and boiler installation where this pipe contains the intake/outlet and bathroom fan extractor. The boiler only needs 5cm plastic pipe (19cm2 area). 1/3 of my 10cm pipe (total 78cm2 xsection) is suitable for the boiler. The bathroom fan may need a bit more power to force the air through the smaller section though.
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How about three square section ducts. Ducting doesn't have to be circular, it only needs to be of a certain internal cross sectional area no matter what shape it is.
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