Your PC must support gigabit ethernet to do this without a router/crossover cable. You'll be able to set up your IP in Windows through network settings in control panel and then adapter settings (if you're on Windows 7). Then adjust properties in the TCP/IP v4 and give your PC an IP of something like 192.168.1.100, Subnet as 255.255.255.0 and default gateway as something like 192.168.1.101.
On the PS3 side, you'll need to do manual network setup and give your PS3 the same IP you put as your gateway on your PC (in this case, it'd be 192.168.1.101), same subnet address as PC and set it's gateway to match your PC's IP (in this case, it'd be 192.168.1.100). Essentially, this will create a mini network between the two devices. Just keep in mind to put your PC/PS3 IP settings back the way they were when you're finished or neither of them will be able to get online.