Well, I look at it this way. They need you more than you need them. Is this dialup, dsl, or cable? If you can get dsl, there are plenty of providers in your area, most likely. The airport supports pppoe, which most people use.
Sharing Internet Service is not like sharing cable. With Cable, everyone who is stealing it can get full use of it. ie. They can watch any channel while you do the same. With Internet Service though, there is a limited amount of the resource, and you are paying full rate for that amount of service, whether you use it or not. Therefore their policy is stupid (IMNSHO (in my not so humble opinion)).
I would call them up, explain that you want to use an airport base station so that you can use your two computers on their network. Here is where you decide what to tell them:
You want to use NAT so you can use two computers, and still be able to talk to the airport (so you want all three devices to be on the same subnet, therefore you need to put the airport on their network, and want to use private addresses on your side.
You want the airport to act as a firewall, you are tired of people scanning your ip address looking for windows vulnerabilities (people do this).
I wouldn't mention the third computer, they don't need to know about it. If they are giving you two static ip addresses, then it is reasonable that one might be a laptop and you would want to use wireless on it.
I would definitely threaten to stop using them as an ISP. Their attitude is very monopolistic. I have changed ISP's for less. Don't let them bully you.
I personally would call them, tell them you are trying to set up a NAT firewall/Wireless Access Point, and the network settings don't make sense. Specifically that a 10 dot (say it that way, 10 dot) address for the default gateway does not make sense for a 194 dot host address. (btw, what are the subnet masks for both of them?) You are not sure how it worked in the first place. If they try to hassle you, threaten to look for an ISP that is more reasonable. If it is a cable modem company, say you are going to look into DirectTV and DSL.
You could try a nicer approach with something like, I have enjoyed my service so far, but this is really important to me.
Good luck, and let me know what happens.
