Doingmybest, I am hearing how it is too long for YOU. Regardless of what might be right for your children, I really believe that our own feelings as mothers/parents are also very important. And if it is too long for you I think that is a totally valid reason for you to ask for them not to go away for 3 weeks. If your children were asked and actually really really wanted to go for 3 weeks, would they really be so upset if it was then only 2 wks? Probably not. Equally with your ex. What is their pleasure v's your pain. I think your pain trumps their pleasure on this one. I don't think them not going for 3 weeks will be as much a suffering as them going away for 3 weeks would be for you. And ditto for your ex. 3 wks would probably be nice for all of them, but 2 wks will also be great. So with more chance of all of you being (somewhat) happy.
I recently went away for a wkend with my husband leaving my 7 and 4 year old with grandparents and I was wrecked by the first night! People may think that is odd or pathetic, but it's just how I feel attached to my children. I literally would end up in a straight jacket with 3 wks away.
Try to find the balance that is right for your children, your ex - but also for yourself.