If you use a Xpath to retrieve the data of B and A you can at the same time set a filter with the "and" command. For example: AttributeOfB = true.
Mariette is correct - you should use XPath to retrieve your data.
To answer in a bit more depth, let's say we have 3 entities:
And a domain model like this:
Contractor * ----Contractor_Company-----* Company * ----Company_Region---- 1 Region
In this case your context / data view entity is Contractor. You can use the "Select..." button to pick a list of Companies for which the Contractor is associated. Then, simply add an XPath constraint like this to filter using a Boolean from region:
If you were filtering on a string attribute, that would look like this:
[Company_Region/Region/RegionName = "USA"]
Alternately, you could simply fill the nested data grid by microflow and pass the top level data view object. Retrieving in a microflow action should give you all the customization options you need.