No, you don't need three partitions.  In some ways having a separate 
root partition is nice, but it's hardly necesary on an new-world mac.  
If you had to have the kernel before the 8gb mark, a boot partition 
might be necesary... and the last I checked you can still use swapfiles 
in NetBSD (altho there's always a chance that's been broken too).

As for the installer, it's been a while for me.. but I don't remember 
being too impressed with its flexibility.  You'd probably have a better  
off just untarring stuff into the NetBSD partition and runing build.sh 
to create an up to date install.

As for sharing the home partition, I wouldn't.  I know that Linux's UFS 
support doesn't handle the new NetBSD UFS superblocks (*SIGH*).. and 
even if you manage to mount a UFS partition from within Linux.. they 
recommend you don't mount it RW, but I seem to remember that the ext2fs 
support wasn't all that great either.  Seems like FAT is the most 
common well-supported FS.

- alex