I am just trying to find real happiness.
What about your children's happiness? You've had three children with this man, then you want to take them away from him because you're bored and looking for something new and more fulfilling?
Sorry, I'm trying not to be harsh here because I can see that you are agonising over this, but it does get me a bit
when people seem to blithely end the relationship with the parent of their children. I'm not throwing stones from my glass house here - I am not with the father of my DD. It was an unplanned pregnancy and we never lived together. Fairly disasterous few years trying to make a relationship work, then split up when DD was two. I've worked bloody hard at keeping things amicable with xDP and we both do pretty well at co-parenting DD - I'd consider him a good friend. But even so, I know it has affected DD, and know it still affects her now (she's 9). She adores her dad, and he her, and very much wishes that she could see more of him than she does. She still fantasises about us all living together in one big house (I've explained that it would have to be a veeeeeeeery big house for me, DP and xDP to co-exist happily
) and I know she is absolutely happiest when she has us all together.
Please don't underestimate the effect that their dad moving out will have on your children (assuming that their relationship with him is good, and that there is no abusive or damaging dynamic between you). It shakes their little worlds, it really does, and anyone who tells you otherwise is in denial.
How do you think it will affect your children having their father move out? Moving between two homes? Rarely, if ever, having both of their parents present at key events? Having to think about mummy and daddy and why they split up, rather than never giving a thought to 'mummy and daddy' other than this entity that's always there, providing for their needs. 
I'm not saying that you should stay in a dreadful relationship 'for the sake of the children', but honestly, I think you should take a long hard look at yourself and what you can do to save the relationship with the man you chose to marry and have three children with. You made promises to him when you married him, and if he is a decent man (as you seem to be saying he is), then he deserves better than this.