On the market I find only Kunai 3 for ROG phone 5. The Italian assistance tells me that it is compatible with the ROG Phone 3. Can someone confirm it for me?
The Kunai 3 was made for the Rog3.
You won't have issues for wireless connection but for may have problems with the bumper as it is made specifically for the ROG Phone 5, so the size and cut-outs are not the same since the ROG Phone 5 have slightly different dimensions (the ROG Phone 5 is just slightly bigger). If possible, try looking for the ROG 3 version, the main difference is logo on it; the ROG 3 version has a purple-ish logo design whereas the ROG Phone 5 version has red wordings and the logo 'ROG Phone 5 Compatible'.
Perhaps the mods can help confirm if it can still work even if the bumper is slightly bigger?
I can confirm that it's still compatible but as Jin_akira stated the cutouts of the bumper case will not match your phone (your fan vent will be covered). But, it still fits and works fine even though it's ROG Phone 5 bumper case. If you don't mind having your vent covered it fits and plays just fine and works 100%.
Wow glad that it still fits and the USB mode is working fine. Good to know as the ROG Phone 3 version is pretty hard to find now with the ROG Phone 5 version everywhere