Try this:
Select all the tasks that are three years behind. Bring up the "Task : Information" inspector and make sure the start dates are not locked. Also, click on the "Reset" button to make the tasks start "As early as constraints allow". This should push your tasks up to the project start date.
If it's still not working for you, send us the file and an email to
omniplan@omnigroup.com and we can take a closer look.