If all items have Docking=Top, the order will be determined by the order they are added in the report's Items collection. In the Context Menu of each you can find BringToFront, SendToBack options which will change the order of items in the report, that can be traced through the Report Explorer
In general, we do not recommend the usage of Anchoring and Docking. Instead you can place the items one after another and they will keep their relative position to each other. To shrink the container you can use the CanShrink feature of R3 2016.
For more details, please check How Report Items are Arranged in a Section
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