Lost 3 audiobooks, how do I find purchase history ?

You can view your account's purchase history on your computer's iTunes : if you log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases.

Do you not have copies of them on your computer nor iOS device(s), nor on your backup of your downloads/library ? Audiobooks are a one-time only download from the store (and other items are only potentially redownloadable whilst they remain in your country's store), so you should be keeping and maintaining a backup. If you haven't, and if they are still in your country's store, you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/