If you open the Calendar widget and go to the second tab page "view" you can fill in the start day attribute. This attribute sets the week in which the day you give as start day as the first week of the calendar In your case you will have to create 12 attributes, where the first one is the start day of the current month, the second one the start day of the next month, the third one the start of the month after that etc. That way you should be able to show 12 calendars for the full year.
In the view tab of the Calendar widget you can set the start date of the current calendar.
See documentation of the Calendar widget: Start date attribute – the DateTime attribute indicating the start date of the current calendar view. Depending on the view, the actual view's start date may differ. For example, if a Wednesday is configured as the start date in a week view then the Monday just before that date is the actual start date.
What you can do is use three dates in three different attributes. When opening the page you could set those three dates where for the first calendar you just use the begin of current month [%BeginOfCurrentMonthUTC%] and for the other two you add a month with the AddMonths function (that Mendix has to offer: https:// docs.mendix.com/refguide/add-date-function-calls) to the begin of current month date . Then use the correct date with the calendars and all three have an increasing start date.