Please try the attached template, which uses JScript to store [current date + 3 days] in one of the form's field when the form loads. To avoid cluttering the form's main data source, you may want to store the value in a secondary data source, though that would be a bit more complicated. Here is the JScript I used, for those who are curious:
// Get today's date
var three_days_from_now = new Date();
// add three days
three_days_from_now.setDate(three_days_from_now.getDate() + 3);
// Select the node where the value will be stored
var targetNode = XDocument.DOM.selectSingleNode("/my:myFields/my:threeDaysFromNow");
// If the node has a nil attribute it needs to be removed
var nilAttribute = targetNode.getAttributeNode("xsi:nil");
if(nilAttribute != null)
{
targetNode.removeAttributeNode(nilAttribute);
}
// Store the date in XML format YYYY-MM-DD, adding zeros where necessary
targetNode.text = three_days_from_now.getFullYear() + '-' +
zeroPad(three_days_from_now.getMonth() + 1, 2) + '-' + zeroPad(three_days_from_now.getDate(), 2);
zeroPad is a simple function that I borrowed from http://sujithcjose.blogspot.com/2007/10/zero-padding-in-java-script-to-add.html. Please see that page to have a look at its implementation