Well... IF you partition your Hard Drive, that means that you erase eeevery single thing, then you go and make partitions using apple's Drive Setup. There you can format one partition to be an a/UX partition (UNIX partition) and the rest you can format to be HFS+, so you dont really need to worry if OS X can be installed on a UNIX partition.
FYI --> OS X can be installed on a unix formated HD, but I dont think that classic will work.
Now I dont know about classic working on ext2 files systems but this seems like the obvious answer... If I am wrong pardon me.
Make 3 partitions. An hfs(+) partition, and 2 ext2 partitons. Then install classic on hfs, osx on one of the ext2's and netbsd on the other. Then install lilo and have osx the default and netbsd the as your other choice.
PS- Make the os x partition hda1. Oh and dont forget to create swap partitions for netbsd (os x too?)
UFS is the UNIX filesystem that OS X supports. ext2 is more of a LINIX filesystem that some unices support, IIRC. (And I may be wrong about the last part, but I am sure OS X won't install on / read from ext2)