I have a program with three groupboxes, each containing a datagrid.
At the moment, only one of the groupboxes and datagrid sizes with the form, and it has been anchored to all four sides.
I'd like it to be all three of them, and also I'd like to be able to size them inside the form with the mouse, if one of the datagrid suddenly contains more data than the other or whatever reason the user might have to size them.

They are placed over/under each other.

If this is unclear I can provide a screenshot.

Thanks in advance.