Once a the marriage is over, your ex-DIL shouldn't be expected to facilitate any visits between her children and her husbands extended family. This is a responsibility that now rests with your son. Married or not, he should have filed for partial custody of his child. And once custody is granted, you as their grandmother can visit with your grandchildren during your sons visitation hours. It would be great if ex-DIL took it upon herself and arranged these visits with you, but you can't expect her to do it, otr get upset when it doesn't get done.
Why aren't/haven't your sons facilitated visits with thier own through a court of law?
If he has not filed for partial custody, then you need to discuss this with him, as opposed to getting upset with ex-DIL and branding the mothers as being spiteful and brainwashing their grandchildren.
I'd further take a moment to re-evaluate and reflect on the situation at hand - if you have 3 grandchildren but don't get see any of them. Again what are all of your sons doing to rectify this? Why haven't they gone to court and obtained a legally binding visitation order? This way the visitaton schedules for the entire calendar year is all planned out in advance, without these expectations that later only lead to disappointments?
Or is there some other underlying issue that led to these family separations that would bar your sons from gaining partial custody/visitation of their own children, and that you as the grandmother are not privy to? Not sure why the blame gets passed on to "mothers," when there are times the men have also quickly moved on with their single lives of moved onto new relationships, don't want to pay child support, have been abusive, simply couldn't be bothered with custody/visitation arrangements etc. This is not an excuse for DIL to be spiteful but it still doesn't negate the fact that it's no longer her responsibility to facilitate visitations with the paternal grandparents.
I feel horrible for you but this is now your sons responsibility. Not unless ex-DIL is violating a granted court order, which in that case your son would have to take her back to court, but she'd now face the risks of violating a court order and loosing custody of her children altogether. So I doubt this is the case. Work on getting access to your grandchildren through your sons.