Adding three redirects for old Directgov PDFs

Approved by the content guys - Zen desk tickets:

https://govuk.zendesk.com/agent/#/tickets/509394
https://govuk.zendesk.com/agent/#/tickets/549886
Merge pull request #568 from alphagov/add_some_directgov_pdf_mappings

Adding three redirects for old Directgov PDFs