Setting the constant that controls the header level for the default layout.

In the constants field (not setup) of your typoscript template add:

content.defaultHeaderType = 3

From that point in the page tree each header assigned a layout of "default" will be an H3. Change the "3" to another number and you will have the corresponding header level (i.e. change it to a "2" and you will have a default of H2).