If you have control of the API, and you know you need your building objects to be fully populated, and your apartment objects to be fully populated, you could expose an endpoint that returns fully populated buildings. That is, buildings with their apartments populated and apartments with their owners populated.

If not, one approach would be to add something like a List<ApartmentResponse> to the BuildingResponse model, and something like a List<ApartmentOwnerResponse> to the ApartmentResponse model.

Then, you iterate through the BuildingResponse models you get back from your API and request the ApartmentResponse models for that building and store it in the List<ApartmentResponse> you add to the BuildingResponse model.

Then iterate through the ApartmentResponse models, request the ApartmentOwnerResponse models from the API, and add them to the List<ApartmentOwnerResponse> you add to the ApartmentResponse model.

For example...

public class BuildingResponse
{
    // Other properties / methods
    public List<ApartmentResponse> Apartments { get; private set; } = new List<ApartmentResponse>();
}

public class ApartmentResponse
{
    // Other properties / methods
    public List<ApartmentOwnerResponse> Owners { get; private set; } = new List<ApartmentOwnerResponse>();
}

public class ApartmentOwnerResponse
{
    // Properties / methods
}

public List<BuildingResponse> GetBuildings()
{
    List<BuildingResponse> buildings = GetBuildingsFromApi();
    foreach (BuildingResponse building in buildings)
    {
        List<ApartmentResponse> apartments = GetBuildingApartmentsFromApi(building.BuildingId);
        building.Apartments.AddRange(apartments);
        foreach (ApartmentResponse apartment in apartments)
        {
            List<ApartmentOwnerResponse> owners = GetApartmentOwnersFromApi(apartment.ApartmentId);
            apartment.Owners.AddRange(owners);
        }
    }
    return buildings;
}

You should be aware that this is a N+1 problem, where you're executing one request against the API to get all the parents (BuildingResponse) then for each of those you're executing another request to get the children (ApartmentResponse). Then you're doing it again for ApartmentResponse and ApartmentOwnerResponse. This could be a performance problem, especially as your list of buildings and apartments grow.