Under the advanced tab you can see what templates you need to override the views template. Copy the templates mentioned from the modules/views/themes folder to you own theme/templates folder, rename them as the view suggests under advanced and style your output.
note: if you display a teaser or full node, but forced it to use fields, the styling (making it h3) won't work.
The h3 is coded in views-view-unformatted-list.tpl.php. The $title there is output of the field you grouped by. There you can set it to h2.