No, I know what you mean about having three children being better if you were to lose one tragically and I completely agree.
I'm the middle of three - my younger brother lost his battle to cancer in June this year. My older sister and I were able to comfort each other and allow our parents to grieve without them worrying how we were handling the loss.
My sister and I talk to each other about how we're handling it, and can talk about how our parents are dealing with it. I live 4000 miles away so my sister is the observer and I"m the talker. She'll tell me if she is worried about them, and I jump on skype and talk them through it. It works well and I know if I'm having a bad day emotionally, I can call her for comfort and not just my parents.
I have one DS who has just turned 1 year old, and I've always wanted three children and oddly, this is one of my reasons. If anything, God forbid, were to happen to any of my babies, knowing they have another sibling to talk to and to get comfort from is a big deal. I don't know how I would have gotten through (and continue to get through) the loss of my brother without my sister.
Having said all this, OP I don't know if there is anything you can or should say to convince your DH. I am in the same position. My DH only wants two, I want three. I would never get pregnant on the sly of course but I'm not about to try and convince him to have three. Also, we live overseas and flying 3 kids about would probably require a second mortgage!
I understand your desire for a third, and I don't have any advice (sorry) - but like you, I can see myself having two and always wanting three to replicate what I had.