The Superkickstart3.1 file is available in lots of places on the net. A regular KS3.1 (for A3000 with SCSI support) with the bonus code tagged on the end.
The file (Kickstart) just replaces the 2.x one ( WB2.x:Devs/Kickstart) and next time you reboot voila KS3.1.
The downsides over a hard ROM are: you loose 512Kbytes of FastRAM and you cannot use an accelerator.
P.S. If you are incapable of identifying and unpacking LZX files, you probably shouldnt have an Amiga at all, not to mention a fiddly A3000 KS1.4 ;-)
