my iphone 4S got replaced again for the 3rd time and now it keeps freezing, turning on and off by itself. what do i do?

When you set up this new device, did you restore from a backup? If so, you may need to set up the device as a new phone, not from a backup. It sounds like you have a corrupt file in the backup.

Just curious, what were the other two replaced for? I would suggest another restore, as a new device, to ensure you get a clean iOS. See if it works like that before you put any other material on the phone.

i really hate to say this but it sounds like you might need to take another trip to apple store ive had to have mine replaced too but only once i know what its like to hope that the problem just kind of fixes itself but never does and then when you take it to the store they talk to you like its your fault.

I can understand being annoyed. It just seems unusual to have the phone replaced again like that and have it end up with problems. I'm not saying that it isn't, but it seems like a reboot loop like that seems like software and not hardware. Hope you are able to get it taken care of. I'm sure Apple will take care of it again for you. Good luck.