Funny because I am experiencing exactly the same thing! ~10C difference between cores 1 and 3. Core 1 always seems to be relatively the hottest core. I was starting to worry a little but I guess as long as all temps are within a good range, I won't focus on the core temperatures so much.
By the way, I am using the Hyper TX3 with the MX-4. Running at 4.2Ghz (-0.065 offset) I am getting more or less the same temps you are. Never over 68C after an hour of P95).