3PX doesn't have an answer for the stealth HK has, he probably could see him, but the issue is that he can't see 360 degrees he'll have to turn entirely to try and find him.
So HK could just pop up from behind and take him out that way. But I haven't read up upon 3PX in depth so I could be missing something, his armoring isn't really special so HK's blaster or grenades should do the trick.
Though 3PX certainly does have the firepower to take out HK, i'll have to read upon 3PX a little more.