Hiya. I've no personal experience here, but the farm where I helped with their Guernseys used to say that the cows produced the same amount of milk overall, just from the three good quarters. However, even if that weren't true, a Jersey who's a good milker will have plenty for three - and she'll make sure her own calf gets plenty. Do you run them all together all the time, or let the calves on for feeding?