Make tablix row or rectangle grow/shrink with content

I hope this is the right place for this post, if not I apologize.

I'm working on a NAV 2015 report and have a question. I have a tablix displaying information about an event. In a row below the main info I have placed a rectangle containing 3 more tablixes (tablices?) displaying data related to the event (food ordered, equipment needed etc). If an event does not have any of this data, this datarow is empty but it remains as high as the height of the rectangle. How can I get the rectangle and the row to grow and shrink according to the content of these three sub-tablixes?

Thank you.