Neither the Books app or the Kindle app for iOS can help you open or read AZW3 books on iPhone iPad. If you have downloaded, saved AZW3 books on iPhone iPad, you can either convert them to ePub file to be read by the Books app or convert them to mobi format and import them to Kindle app on iPhone. Calibre is a free ebook converter can help us with that. Check out below steps to convert AZW3 to Mobi, ePub for iPhone or iPad using Calibre on computer for free.
Send books from iPhone to computer
You can use a free file transfer app such as Xender, Zapya, SHAREit to transfer the books from iOS device to computer or vice versa. Check out this guide to transfer Mobi, ePub books from iPhone to PC. It can help us move AZW3 and many more file types.
Download calibre on your computer
Calibre is a e-book converter and management software for desktop. You can download and use it on your Windows PC or Mac computer. Do a search for calibre in Google, it will come at the top of the search result. Download and install Calibre on your computer and follow below steps to convert AZW3 to Mobi, ePub for iPhone or iPad using Calibre on PC or Mac. We will use Calibre for Windows in this demo.
Convert AZW3 to Mobi, ePub using Calibre
Run the free AZW3 converter on your computer. Click Add books button at the left end of the top toolbar, browse to the folder where the AZW3 books are saved on your laptop or computer and import them to the book converter program.

Optionally you can hit the Edit metadata from the top toolbar to edit the metadata of the book, including title, author, title sort, author sort, cover, rating, tags, published date, publisher, language and so on.
Click Convert books button at the top toolbar, then choose the format you need from the Output format box, such as EPUB, MOBI, DOCX, etc. Here you have another chance to edit the book metadata. Finally hit the OK button at the bottom right to convert the AZW3 book to Mobi, ePub or other format of your liking for iPhone or iPad using the free ebook converter Calibre on your PC or Mac.

Send books from computer to iPhone iPad
You will be taken to the home interface of this free ebook converter. Wait for a second for the AZW3 books to be converted to Mobi or ePub. Once the job completes, hit the Path: click to open link to open the output folder where both the original book and the converted book in new format are saved. You can then send it to your iPhone, iPad through email or a file transfer app. Scroll up to the section where we discussed how you can send books from iPhone to computer or the other way around.