The grid views of the calendar (main and mini) don’t support showing multiple months at a time, so there isn’t really a great way to do that. You could use three successive widgets, using the +1 and +2 modifiers for the date selection in the additional widgets, however.
Okay – where do I find the the date selvtion so I can put in the +1 and +2 – and does this also work with shortcode generator ?
It’s the setting labeled ‘Mini-Calendar Timespan’ in the widget, and yes, it can be used in the shortcode, though only ‘next month’ is an option in the generator. Just iterate the value from month+1 to 2 or 3 to adjust it further.