Use "t" instead of 'd" to get the last day of the month, and start with the first day of the month when adding the months like:
$InsertQuery->bindColumn("EventDate", "t", "".((isset($_POST["datepickerDefault"]) && $_POST["datepickerDefault"]!="" )?date("Y-m-t", strtotime("+3 month", strtotime(date("Y-m-1",strtotime($_POST["datepickerDefault"]))))):"") ."", "WA_DEFAULT");