To Partition:
Disk Utility, Select drive on left side, Partition, Select 2 Partions under Volume Scheme, Input name for first drive (Time Machine), then click on bottom box and name it something like PS3 Backup, click Options, click Master Boot Record (note it says it's for Windows or DOS machines), click OK, click Apply.
Once the drive has been partitioned and formatted then:
System Preferences, Time Machine, choose the disk you just created and begin the backup. This initial backup can take hours depending on how you have the Lacie connected. You never mentioned the connections the Lacie has, if it has Firewire 800 then I would suggest using that for your TM.
Having never owned a PS3 I don't have a clue how to connect it. I would imagine that would be a drag and drop process though. If your iMac can see the PS3 then that should work fine.