setting up triple boot Win2k, Winxp and Win98se:

I had a chinese game that will only run in win98, but I'm currently running dual boot Win2k/Winxp.

Current config:
C Drive => Win2K installation
D Drive => WinXP installation

I don't want to delete the installation to install win98 from scratch, so if I add a third hard disk drive as E Drive, how can I install win98se to E and make it a triple boot system?

Okay, install 98 on that new drive just like you would normally.
Then you'll notice that you can only boot into 98, even after the install is complete.
Now you have two options: repair the install with the Win2k disk or the WinXP disk.Either way, you will have to boot off the cd like you would to install it.
For WinXP: just follow everything like you would to install, but then when you see the options to repair, install or quit all together, choose the repair option. You may need to reactivate afterwards.
For Win2k: you'll see the Install screen and have the option to repair it also, which you should choose. Do the fast repair and the option to have the disk find the Win2k install itself.

Ideally, the best course of action would've been to install 98, then 2k, then XP, but it should still work fine.
I'd suggest running the XP repair over the 2k repair, because it's a little less hair raising after you get used to it.