My proccess was like this:
1) HR called to offer the position
2) Few minutes later I received 3 e-mails
I. The email with two attachments, a tentative offer letter [word doc] and a PDF with the PHAP general info
II. USAJOBSTAFF invite to with login that expires within 72 hrs
III. USAJOBSTAFF with the forms that needs to be complited, an the webpage that you will need to login to fill out those forms.
Did you have the same structured emails?
What does your 3-page letter contains?
I don't think it is a good idea to find the form online and fill out. You should do it under the specific wage page to make sure the HR has received it.
Double check your emails. The email expires within 72 hours. If that is the case for you, e-mail the HR and she/he will reactivate again for you.
You should find all the forms in this web page.
https://onboarding.usastaffing.gov one you have your login info