I wasn't sure either. Then I went to the Apple page. There, the Logitech Combo 10.5 (actually for iPad Pro 10.5) is shown as compatible with the 10.5 iPad Air 3. To be on the safe side, I ordered it directly from the Apple page. It has just arrived and everything works perfectly.
Since the iPad Air3s have been out for a while, I recommend that Galaxus add them to the compatibility list in the shop.
I don't think I will ever do without a keyboard on the iPad again. It's already a lot of fun.
Yes, that's a bit of a difficult question, isn't it? I have the case on my iPad Pro 10.5 and am totally happy with it. You could come to me in Baden or Zurich with your Ait and try the case on :-)