You have the option during to install FreeBSD's boot loader at all. If you let it install by accident, you can just use the RH CD to boot to redhat and then use the "repair" option to re-install grub with your old grub.conf file.
You can configure FreeBSD's bootloader to do linux and win98. Offhand, I don't think its just going to "automagically" detect the other two.
To find the proper BSD stanza to add to grub.conf, just do a search on www.google.com/linux
for "grub and FreeBSD" and you should be fine.