Prepopulate calendar field

is there a way to prepopulate a calendar field to show 3 weeks in advance

When you add DateTime field inside of your form you have an option Default date. Then selects Custom date and there you can enter any day you want. 

Here is my demo form where I set custom date 3 weeks form now.

http://form.jotformpro.com/form/51615736463963? 

Hope this will help. Let us know if you need further assistance.