FF, I'm sorry you are here. However, if you could possibly keep to one thread, readers can then follow along with your postings and be more of a help to you.
About the spouse walking away from the children, if you continue to read the postings all over the forum, you will see that you are not alone in this mess. Many of them do walk away from their children and do not look back for a very long time, if ever. There are some here who have been fortunate in the fact that their spouses still spend time w/the children. But, over all, many walk away to rediscover the world. If your h is in mlc, it will take quite a bit of time before he makes contact w/them again. Three years is a long time, but what makes me think he's in crisis is the fact that he's not even told his family where he's living.
The comments he has made about the children are spot on. You are absolutely correct in assuming that the youngest cannot judge him and that will be the child he will reconnect w/first..this child will become his pal, his buddy. The other two are competition for him. You have to remember, if he's in crisis, he's acting out and will be about the same age as your two older children. Again, it takes a long time for the mlcer to grow up (in our eyes).
When and if he grows up, he will regret what he's done to his children. He's the one that will need to figure out a way to bridge the gap of not being there for them. Whether or not the gap is bridged will depend upon your children. Some children get to the point of speaking and/or not speaking to the parent that walked away. The respect and love tend to go by the wayside when they do not have contact w/their children. Unfortunately, there's nothing you can do about the situation...he has to fix it.
FF, all you can do is be the best parent you can be. Love your children, be there for them and show them that you do care, even when you are having a bad day. You are the lucky one...you have your children who are there w/you. What does your h have? A ow that doesn't amount to beans. Hold your head up high, do not allow his comments to others sway your thinking. For what he's saying are just excuses to justify why he's out there doing what he is.
You are the only one that can decide whether to stick it out, take him back if he returns, etc. Again, read some of the other threads on this forum...you will see that you are not alone. Many of them are struggling with issues of the spouse abandoning the children. You just may find some pearls of wisdom on the other threads.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.