All the cores will be about the same temp it dose not make a difference if it only shows 3 of them, at most 2c difference
IT doesn't show 3 of them - it shows all 4 but they all read 0C all of the time - when you unlock the extra core it makes the individual core sensors no longer work !! - the only monitoring programs that I have seen still work are Everest Ultimate and Easy Tune 6 which report the CPU temp correctly (HWmonitor does not have a report for the CPU temp and just has the individual core temps which are always reported as 0C after unlocking !!