The Pentax K-3 was supported for the first time by ACR 8.3 which requires at least Photoshop 13.x (CS6) or higher.
As CS5 is no longer supported, it will never get an ACR update. You will never be able to open raw files from that camera directly in CS5.
You need to update to at least CS6 (CC will work too of course), or you can use the latest version of the free, stand-alone Adobe DNG converter with which you can convert your PEF files first to DNG files that yopu can then open in CS5—as long as you don't mind nissing the superior rendering and other features of ACR 8.3 and higher. Your choice.
Can't the Pentax K-3 save images as DNG? That would probably be your best bet if you stick with CS5.
Most Pentax DSLRs since the K-20 can. K-3 is newer. Good point.