Make that the vents are completely clear, ie no games/walls/clutter nearby. I noticed that my PS3 frooze a lot more when my room was hot, after hours of gameplay, or when something was close to the vent. Meaning that if the PS3 gets hot, it is more likely to freeze. I play BF3 all the time (add me on battlelog if you want BUNDYnMANSON) and only occasionally experience freezing problems. Although it has happened to everyone that I know who plays it, which means that it is likely a problem with BF3 and not with your PS3. There are tons of bugs to be patched in BF3, so stick in there and wait a few months until the developers have had time to figure things out (frustrating, I know).