See the wiki ->
https://wiki.mobileread.com/wiki/Kobo_Firmware_Releases
Updates are fairly regular and newer firmware seems to work on older devices too.
But I think the biggest draw may be to install koreader which actually has better functionality than either kindle or kobo software ->
https://github.com/koreader/koreader...n-Kobo-devices
EDIT: if you have a paperwhite 3, you already have a current gen eink screen -
there is no need to switch if a better screen is your only requirement. Did you mean a regular kindle 3 instead of paperwhite 3?