No, it does not have to be an Apple Airport card. This is just as well as there is nowhere to put one anyway.
You will need a wireless PC card that is compatible with Mac OS X.
Do a Google search for such a device. Apple do not make a wireless card for the G3 laptop range.
Check out this site for more information:
I just purchased my first mac a cheap used g3 lombard. I've been wanting to try a mac and the price was right. It has OSX on it, can I use any wireless card to connect to my wireless network or does it have to be an apple aircard?