In order to open source Amiga OS 3.x one would need the source.
Check you history; The entire source code for 3.x does not exist. It went missing somewhere in the Commodore labyrinth. This was one of the issues faced by the early AmigaOS 4 team. There was no "gold" code stored in a source repository. Bits and pieces here and there. Decompile this and reverse engineer that.
That's not completely true. AFAIK, parts of the source code for AmigaOS 3.5 and 3.9 weren't available to the AmigaOS 4 team (e.g., the fancy CD player, updated installer), but the source code for AmigaOS 3.1 was.
Here's a video generated from the AmigaOS source-code repository showing development from AmigaOS 1.0 in 1985 through to AmigaOS 4.1 update 2 in 2010. Clearly Commodore's source code repository did survive its demise.
Hans