I think that this is a pretty personal question and that there isn't necessarily a 'right' answer- maybe not even an answer that applies to all three of your children. I can confidently say, though, that I don't think you should ban any of your children from being in the room when a loved one passes, unless that's the desire of the person who is dying.
The first thing to consider is what the child's perspective and temperament is- do they seem scared about the situation, do they have some understanding of what is occurring, do they want to be in the room or are they too upset? Are they old enough to remember the event or remember being excluded from the experience? If they're old enough to remember and they want to be there, I absolutely think they should be allowed. There's nothing like the feeling that you didn't get to say goodbye to a loved one- they'll be angry, resentful, and feel guilty all of their lives. I think children's emotions are often treated as less important or less powerful than adults', and it's really important to acknowledge how powerfully children feel, especially about something as major as losing a loved one. To deny them access to their parent or grandparent while he or she is dying might make them feel like their grief is being treated as less real or important to deal with than that of the other family members.
Second, can you handle it? Are you (or another important adult) in a position to care for and pay attention to them while the death is happening or are they going to be watching someone die and feeling ignored/alone? Are you prepared to explain beforehand, pay close attention to their needs during, and help them process the experience afterwards? A child shouldn't be expected to deal with a death that is never explained or discussed afterwards- if they are going to be present it should clear that they can talk about and work through their feelings afterwards as often as they need to. The other thing here is that I think that if an important adult in a child's life is dying, the child should be with another very close adult caretaker- so if the whole family is at the hospital or wherever the child should be with them, not with a babysitter.
The first time I lost a close loved one I was 12. My grandfather died in a pretty horrific and totally unexpected accident in the woods, and while I wasn't there when he died, I did see his body in situ shortly after (before it had been moved or cleaned up at all). It was a terrible shock, but I am so thankful that when my family went out to see him I was allowed to come along- that was when I really processed the fact that he was dead, not when I saw him looking so artificial and posed at the funeral home. I did have some bad dreams afterwards, but I think of that night as an important one in my life. I know that I would have felt excluded from an extremely powerful family experience if I hadn't been allowed to see my grandfather's body the night he died.
In general, I'm a pretty big proponent of including your children in every aspect of normal life- both the positive and negative parts, so if I had to boil my answer down, it would be that I think children should be present if someone they love is passing. It's just important to make sure that they have lots of context and support both before and after the event.