I wrote a guide on compiling the kernel for Kindle 2 here:
https://www.mobileread.com/forums/showthread.php?t=91862
(I also linked to a pre-compiled toolchain)
The only differences for Kindle 3 is:
1) Download the Kindle 3 source (duh)
2) For step 2, instead of "make mario_mx_defconfig", do "make imx35_luigi_defconfig"