OP you say that the GC are 6, 5 and 2, presumably you've looked after each of them as they've come along, and haven't suddenly been introduced to them, as 6, 5 and 2 year olds?
Assuming that's the case, then I don't really understand why you haven't already instilled rules about how they behave when they're at Nanny's? I too have 3 grandchildren, although all grown up now, but I used to look after them quite a bit from the time they were babies, and so they grew up knowing what was acceptable at Nanny's house, and what wasn't. In fact one Christmas Day their parents left them with me for a couple of hours while Mum went home to get dinner organised, when they came to collect them, we were sitting at the table doing some crafting with their new things. Their Dad stood and observed for a few minutes, and then said "How come they behave so nicely for you, and yet are little horrors at home?" The answer was "I don't allow them to get away with bad behaviour, it's as simple as that". So while I do understand your complaints about their behaviour, I can't help but wonder, why you've allowed them to get to this stage when they're with you, as if you've kept them in line up until now, what has changed? Or have you also permitted their bad behaviour up until now, and are suddenly beginning to see that indulgence when they're young, leads to poor behaviour as they grow older, and have realised that you don't like it?
If you don't want to fall out with your DD, I think your best plan, is to sit the bigger children down, and tell them firmly that their behaviour is upsetting you, and that in future, if they're naughty, or rude / ignore you, etc., then they will be punished, and MEAN IT! Decide in your mind what are suitable punishments for them at the ages they're at, warn them when they start to behave badly, and then implement the punishment if the bad behaviour doesn't stop, but whatever you do, don't make the mistake of threatening something you're not prepared to follow through with.
On the other hand, if you've genuinely had enough, then do what most Grans are advising, and tell your DD that you are getting too old to cope with 3 boisterous kids, and that she will have to find alternative child care.