Hi Atse, you might consider a ListView widget. This widget provides the show more functionality you descibe.
You might also consider putting 2 grids on your page. One would have just the 3 lines, and other would show 20+ rows with paging enabled as Mike suggests. Then, set conditional visibility on these grids based on some boolean attribute in your context entity, and create a "show more/ show less" button that toggles that attribute's value.
In the properties of a DataGrid, there is a switch which will display paging buttons, shown below:
This will display the following paging buttons on a datagrid:
Not sure if this meets your requirements or not.