preggydonuts if this thread makes you feel worse, leave it
As I see it, four adults (including you) have already been badly hurt. I think your ex is extremely hurt and it sounds like OM's wife has been hurt badly for years. You are hurt and I think even OM is. Your 3 kids have been through the break-up of your marriage, OMs kids are picking up at least on the tension at home and their dad being thrown out or leaving twice.
There has to be an end to all this somehow. For now, I would try to concentrate hard on protecting your baby who is in no way to blame for any of this. If OMs wife calls again, why can't you gently tell her the truth and if she goes mad, say "I'm sorry for all the hurt to you and your kids but I have to hang up now" and put the phone down. Other than that , keep away from them and keep yourself as calm and strong as you can. Your baby doesn't start experiencing things when he/she is born, your baby is going through all your emotions right now with you as you go through them - sadness, anger etc all that.
Have a little think and try to give your baby a calm mum to live in till s/he arrives, be honest to OM that you are keeping the baby and keeping away from a relationship with him but you welcome him as a father to your dc; tell his wife if she calls the truth if she asks, apologise and tell her that you will not be continuing the relationship but you are keeping the baby.
Tell ex that you are keeping the baby, not maintaining the relationship to OM and apologise for the hurt to him and reassure him that you want him as your dc's dad in your lives and you'll do what you can to make him feel welcome.
Then get into preparing in a positive way forthe coming of dc number 4. You have some time to get emotions sorted and calmness back in the picture before the birth but you have to make up your mind to do it and then buckle down to being consistent with it
That is what I would try to do