how can i find my old documents when my 3rd generation ipad has been updated?

Safest bet, on the backup you carefully performed before doing an update.

Under normal circumstances, an update does not touch app documents. However, there is no guarantee that will hold always, hence the recommendation to do a backup before any update. Heck, if you do the update using iTunes on a computer, that program will do a mandatory backup before proceeding.

So, if you have updated everything, including all the apps, and no document is visible, there's a fair chance something went wrong along the way and they are lost. Updating all apps is particularly critical when updating the iOS to 8.4.1. A large portion of that update are important security fixes, which may cause older and less secure apps from continuing to work in their unsafe ways.