Here's a way you might understand it:
You have 3 computers. One is apparently going to be wired straight to the router with an ethernet cable.
The others are going to be linked to it through radio signals (wireless cards).
Try to excuse the fact that one is physically connected to the router - they all will actually be connected similarly.
What I am saying is that it doesn't matter which computer is plugged directly into the router, they all will be connected to it.
Windows98 will allow you to share folders and printers, and you will be able to access these from other operating systems.
The Windows98 computer will not be sharing internet with the other computers, it is the router itself that is doing that.
Thus, it makes no difference which computer, if any, is cabled to the router. They may all be connected wirelessly if you choose to do so (although not cost effective). Internet will automatically be accessible by any computer connected to the router wirelessly or through ethernet cabling.
Also, for Internet and networking's sake, make sure you have your cards set to automatically obtain an IP address. The router will assign a class C network IP (ex: 192.168.0.X) to each computer. The computer will then be able to access the Internet that is coming into the router from the modem. Also, the 3 computers will be able to 'see' each other in My Network Places/Network Neighborhood, allowing you to transfer files and share printers.