@topquark: Will what work on K3?
1) FONT_RAMP thing >>> No, it's firmware 5.x.x series only.
2) Reducing left and right margin >>> IIRC it's easier to do that on K3 than on any newer Kindle (you edit a value in /system/reader.pref file that is just a plain text file and accessible without needing the jailbreak
3) Changing those 8 predefined font sizes in Aa menu: Possible with jailbreak by editing different set of jar files.
See this post for hack/solution for 2) and 3):
https://www.mobileread.com/forums/sho...&postcount=137
You should probably read at least last couple of pages of that thread, focusing on posts by @ihor. And also read the included "readme"/instruction to check which firmware build for Kindle 3 are supported and tested. Especially now that Kindle 3 recently got some small update (for March 22 server side changes) to be able to continue using Kindle Store and stuff... because there is possibility that those jar files that hack uses were replaced and/or java code was re-obfuscated.
Having working SSH setup (USBNetworking) and some basic knowledge of Linux terminal command would be nice, in case you just go ahead with installing the hack and as result Kindle gets soft-bricked you could manually delete modified jar files... that should fix things.