If you download a BR Rip from the internet and find some way of transferring it to the PS3's HDD then it will play it. Bear in mind, that you can not play them from External HDDs or USB sticks and so forth as for external drives, the PS3 can not read any files about 4gb due to the FAT32 restrictions.

If the files are MKV, the best thing to do is just stream them straight from your PC using TVERSITY. Then you dont have to mess around transferring shed loads of files to the PS3.