Actually, you can. However, you shouldn't.
Right now your computer assigns another IP address to your ps3. If you set it up so your access point was connected to the modem, all of a sudden your modem would be assigning two IPs. And not all services provide more than one.
If you want to give it a shot anyway, log into your access point, and give it a static IP address. Then plug it into the modem. Unplug power from the modem for about a minute, then plug it back in. Now boot up your computer and connect to the access point (assuming you've got a wireless card in your computer.)
You should be able to connect at that point, as well as have your IP assigned by the modem. Whether or not you can connect your ps3 at the same time, is entirely up to your ISP, and how many IP addresses they assign to you.
Really though, it would be in your best intrest to just go buy a router. It doesn't need to be wireless as you already have an access point that you can configure to work with any non wireless router.