Best answer: Yes, cases for the 10.5-inch iPad Pro fit the 10.5-inch iPad Air 3, including Apple's Leather Sleeve and Smart Cover lineup.
- Maximum protection: Apple Leather Sleeve ($129)
- Smarter: Apple Smart Cover ($49)
It fits!
If you have a Smart Cover from the 10.5-inch iPad Pro released in 2017, you will be able to use it with the new iPad Air 3. Or, if you have a sleeve-type case that is designed to fit any tablet, you should be fine.
Unless you're upgrading specifically from the 2017 iPad Pro (10.5-inch) to the 2019 iPad Air 3, you'll be in the market for a new case. The good news is that you don't have to wait for new cases to come out. Most 10.5-inch iPad Pro cases will fit the 10.5-inch iPad Air just fine.
Keep in mind, however, that the 10.5-inch iPad Pro has four speakers (two on each side), while the iPad Air 3 has two. So any cases explicitly made for the 10.5-inch iPad Pro might have an unnecessary second cutout or grill for speakers. Plus, the 10.5-inch iPad Pro has True Tone Flash, while the iPad Air does not. So, any camera holes are going to be bigger than they need to be.
Since the Apple Smart Cover only covers the screen, not the back of the iPad, the slight differences in the camera and speakers won't matter. Apple notes on their site that the Smart Cover for the new iPad Air 3 is also compatible with the iPad Pro (10.5-inch.)
Smooth leather

Apple Leather Sleeve
Security at its best
The official Leather Sleeves offer protection for your iPad and Apple Pencil.
Cover it

Apple Smart Cover
Colorful choice
Smart Covers for iPad are typically available in various colors, including seasonal versions. What color will you select?.
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