If your machine has a disk and network connection, but no tape drive, it may be convenient for you to install NetBSD over the network. This involves temporarily booting your machine over NFS, just long enough so you can initialize its disk. This method requires that you have access to an NFS server on your network so you can configure it to support diskless boot for your machine. Configuring the NFS server is normally a task for a system administrator, and is not trivial.
If you are using a NetBSD system as the boot-server, have a look at the diskless(8) manual page for guidelines on how to proceed with this. If the server runs another operating system, consult the documentation that came with it (i.e. add_client(8) on SunOS).
Your Sun3 expects to be able to download a second stage bootstrap program via TFTP after having acquired its IP address through RARP when instructed to boot "over the net". It will look for a filename derived from the machine's IP address expressed in hexadecimal. For example, a sun3 which has been assigned IP address 130.115.144.11 will make an TFTP request for `8273900B'. Normally, this file is a symbolic link to the NetBSD/sun3 "netboot" program, which should be located in a place where the TFTP daemon can find it (remember, many TFTP daemons run in a chroot'ed environment). The netboot program may be found in the install directory of this distribution.
The netboot program will query a bootparamd server to find the NFS server address and path name for its root, and then load a kernel from that location. The server should have a copy of the netbsd-rd kernel in the root area for your client (no other files are needed in the client root) and /etc/bootparams on the server should have an entry for your client and its root directory. The client will need access to the miniroot image, which can be provided using NFS or remote shell. If using NFS, miniroot.gz must be expanded on the server, because there is no gzip program in the RAMDISK image. The unzipped miniroot takes 8MB of space.
If you will be installing NetBSD on several clients, it may be useful to know that you can use a single NFS root for all the clients as long as they only use the netbsd-rd kernel. There will be no conflict between clients because the RAM-disk kernel will not use the NFS root. No swap file is needed; the RAM-disk kernel does not use that either.