Hello and welcome Gecko,
it seems splitting buildings in the iD editor is quite hard for beginners (see: Splitting a Building with iD)
Though to my knowledge I would say that @GA_Kevin is right and since it all shares one roof and foundation I would indeed classify it as one building, too.
The easy way is to add nodes inside this building, that contain all the information differentiating the different parts. If you want to go the route of really denoting where one part ends and the other begins, you can use building:parts(Key:building:part - OpenStreetMap Wiki). The latter workflow would mean making a new area and tracing the outline of the big building until you reach the line of separation. That way you would have 3 areas that you can give different information.
(To make the latter easier, I would remove some points from the curves, since they are excessively accurate (I could prep this for you in JOSM if you want))
(Also the whole building seems out of place since ESRI are the ones with orthogonal imagery, so I’d use that)
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