(1) you uploaded version 1.0, which was based on the Add-on SDK
(2) you uploaded version 2.0, which was based on WebExtension
(3) you uploaded version 3.0, which appears to be using the Add-on SDK again
From DaveRo's link:
In other words: Make sure that version 3.0 in the above example is also a WebExtension Add-on. In case it really/already is a WebExtension, try to figure out what makes AMO think that it isn't.
I didn't upgrade my SDK Add-on to WebExtension yet. But it looks like AMO should display a warning when you upload a WebExtension version to a legacy/SDK Add-on the first time: Just to make it abundantly clear that all future versions must also use WebExtension.