The app's name is iTerm2. Version numbers have to increase lexicographically, so the version 3 beta is called 2.9. I'm open to suggestions :)
It also allows you to add a suffix though for beta-type versions, so the beta would be v3.0.0-beta, or v3.0.0-rc1, etc.
Versions are supposed to convey meaning over how much has changed to the app between two versions. iTerm got soooooo many new features in this new version, anything smaller than a major bump would feel out of place.
I do agree the actual version should be v3.0.0-beta1 tho ^^.
SemVer is for APIs. From the spec:
> 1. Software using Semantic Versioning MUST declare a public API.