Does it support EPUB3?
Yes. From their repo README:
https://github.com/johnfactotum/foliate
Supported formats:
- EPUB (.epub, .epub3)
- Kindle (.azw, .azw3) and Mobipocket (.mobi)
- FictionBook (.fb2, .fb2.zip)
- Comic book archive (.cbr, .cbz, .cbt, .cb7)
- Plain text (.txt)