Tweaking GRUB
 

I would like to have W98, FC3 and Slackware 10.1 on a single hard drive. W98 would occupy a primary partition, and FC3 and Slack partitions would all be resident in separate logical partitions within an extended partition.

When I install FC3, it overwrites the MBR with GRUB. How do I go about editing GRUB such that I can select any of the three operating systems to boot?

use VI (or any editor) and open /boot/grub/grub.conf. If that life doesn't exist, then it is likely called /boot/grub/menu.lst

Since you didn't post your partition scheme, what I type probably won't work but should give you a rough outline.

The entries would look something like this:

title WIN98
rootnoverify (hd0,1)
makeactive
chainloader +1

title FC3
root (hd0,2)
kernel /kernel-2.6.10 real_root=/dev/hda3

title Slack 10.1
root (hd0,3)
kernel /kernel-2.6.10 real_root=/dev/hda4

musicman,

I appreciate the pointers. Give me an example of what you mean by a partition scheme, and I will gladly post mine.

partition scheme
 

I think he/she meant the output of
post this and we'll try to help you. Or you can just replace real_root=/dev/hda4 with the needed partition.
if you have windows on 1st partition, then the win menu entry would look like this:
and if you have slack on partition nr. 2, then slack menu entry would look like this:
NOTE: pay attention to real_root=/dev/hda2 and (hd0,2) - these are important parts. Also assuming here, that you are using kernel 2.6.10 here,
and it is located at /kernel-2.6.10.

Hope this helps!