This assumes you will be starting from scratch and will reformat/repartition. A bit of background on microsoft disk formats and which OSs understand which. 

WindowsNT FAT16 NTFS 
Windows98 FAT16 FAT32

So it makes sense to use FAT16 for a dual boot system (at least for the partition that the OS resides on) because both NT and 98 can understand it natively. Linux can mount FAT16 and FAT32 with smbfs which has been incorporated into the kernel. You could also make NTFS or FAT32 partitions, but they will not be shared by both OSs. The order of this process is critical. Installing Win98 first then NT makes the NT boot loader automatically configured for dual boot. Then installing Linux last allows you to select between Win98/NT and Linux. 

___Partition and Format____

Partition using DOSs FDISK. It is easiest to boot off of a Win98 setup floppy and start FDISK from there. When questioned if you want large disk support, choose no ('N') to ensure that you will partition and format FAT16 and not FAT32 partitions. Partition disk as you like. I would reccommend making a primary DOS partition to install both Win98 and NT on and leave the rest as an extended drive to use for other logical drives and for Linux. (If you have a disk of size greater than ~8GB you have to make sure that the Linux boot partition lies completely below the 1024th cylinder of the drive. See "Appendix for Large Drives" below for more info.) 
Here is how I partitioned a 6.4 GB drive: 2GB Primary DOS (C:) [ 2GB logical DOS drive in Extended DOS wrapper (D:) 2+GB unpartioned free space ] You will have to reboot after partitioning, do so. Boot again, using the Win98 setup floppy. Format the partitions you created using DOS's format (i.e. 'format c:') 

___Windows 98 Installation___ 

Boot off the Win98 CD and install it to the primary partition (or type 'setup' at the DOS prompt if your BIOS doesn't support booting from the CD). Install Windows 98 with the options you would like. I installed Win98 into the default C:\WINDOWS directory. 

___Windows 98 & NT testing___

Now you should have a dual boot machine with some free space in the extended partition. Try and boot both OSs. The OS Loader should have autoconfigured itself to recognize both Windows 98 and NT. Play around a bit. Pat yourself on the back for making it 2/3 of the way through! 

___Linux Installation___ 

Now install Linux. There are a billion books and HOWTOs on this subject so I will keep it very simple. I will also wuss out and outline the simplest install of the simplest Linux distribution. That is, I will use the case of RedHat. I installed RedHat 6.0 onto my system. Make sure you have all the info you need (IP #, possible monitor refresh rates, video card type & amount of video RAM, network card type, etc.) Boot off of the RedHat CD (again, use the appropriate boot disk if your BIOS doesn't support it). 
Use Disk Druid to partition your drive, it is very simple and nearly intuitive. Don't mess with the drives you have already formatted, ESPECIALLY the primary DOS drive, unless you want to. Use the free space to create at least a '/' partition and a swap file for your Linux setup. It will then partition and format the disk for you. Choose what kind of install you would like. 

I usually do a custom install, where you can choose exactly which packages get installed, but doing a 'workstation' or 'server' install may save time. [NOTE: The RedHat server install uses ALL disk space available and reformats the drive, while the workstation install uses all available unpartitioned (or Linux partitioned) diskpace. I guess that after having goe to all the trouble of having followed your excellent partitioning instructions, people are gonna be less than pleased to have their new partitions wiped by a server install.--thanks for the heads up on this Steve]

Start the installation. Isn't that nice? After the install is complete, the installer will configure a few things. Among them are your mouse, monitor, graphics card, and network card. Setup all that stuff and give info where needed. Make sure you remember the root password you chose!!! It is also a good idea to make a boot disk. If you re-install Windows, you will need it to recover LILO and access your Linux boot. 

One of the last things will be to setup LILO. This is crucial. Choose to install LILO onto the master boot record (MBR) of the main drive. It will then ask you to choose the labels for the OSs to boot to and which is default. There should be two options "dos" & "linux" and linux will be the default. I used 
to change dos to the default and then used edit to rename the labels to "win" and "lin". Your system will reboot at the end of this all. Remove the boot disk you created. You should first see the following prompt: LILO: If you hit you will see your options. Otherwise just type one of the labels and it will boot into that OS. If you choose your linux option, you will boot into Linux. Otherwise, if you choose your windows option, you will then get to the OS loader which will ask you which windows (98 or NT) you want to boot. Ergo you now have a triple boot system! 

___Appendix for Larger Disks___ 
If you have a disk which has more than 1024 cylinders even after being mapped by the BIOS using LBA, then you have to worry about where you put Linux. This occurs somewhere around the 8 GB mark. Lilo and the boot partition must reside below the 1024th cylinder. 
Here is an easy way to do this. 
Partition something like this (assuming a 10 GB drive): 
2GB Primary DOS (C:) 
[ 2GB logical DOS drive in Extended DOS wrapper
(D:) 2GB logical DOS drive in Extended DOS wrapper 
(E:) 2GB logical DOS drive in Extended DOS wrapper
(F:) 2GB logical DOS drive in Extended DOS wrapper
(G:) ] 
Format all the partitions as FAT16. Install Windows 98 and NT onto the primary partition. When you install Linux, use DiskDruid to delete the first logical drive and repartition it as your '/' and swap space. Now just install as normal. This will ensure that Linux will reside below the 1024th cylinder. You can always change some of the partitions to FAT32, NTFT, or ext2 if you like later. 

-Barron K.