So you want to install Windows NT, 98 and Linux on a 20Gb Hard disk.
First structure of partition should be like this:
Primary Partition 1 - Win 98
Primary Partition 2 - Win NT
Primary Partition 3 - Linux
Primary Partition 4 - Linux - SWAP
The size for above partitions are for you to decide, for SWAP I recommend 2X the size of your RAM.
The installation order should be different too.
Install 98 FIRST on partition 1
Install NT SECOND on partition 2
Install Linux THIRD on partition 3 (NOTE: Partition 4, you will set that up during linux installation )
You can partition the hard disk using FDISK utility, which is found on Win98 startup disk, CDROM, NT etc...
If you have anymore questions, just post here :-D
Cheers