Ah, I hate that happened to you. I could see having one calve early, but not three without there being some underlining cause. It could be the genetics of the bull you haven't bred with before even though he comes from good stock. But it could also be due to heat stress and other weather extremes which puts too much pressure on the heifers. Did you feed them pellets? I remember reading an article a year or two ago that said something about a toxic level of ergot being found in some food pellets, but I don't think they concluded that it caused the premature births, just said it could have contributed to it. Did the vet say why he suspects disease could be the culprit? I thought diseases, especially ones that could cause premature birth, were pretty uncommon in a well-vaccinated herd. I hope you get to the bottom of it.