It seems to me your wife is more worried about losing you than sharing you. Or rather, she's afraid the sharing will make her lose what was so special about your relationship.
You might be able to explain things to her based on your kids... When your second kid was born, the first one didn't become any less special and loved. Indeed, the second one probably made you appreciate the uniqueness of the first one due to all the things that were different. Same thing with the third one.
Your wife is her own person, she is unique and so is your relationship with her. J isn't "more of the same" or "a replacement", she is a different person who could never bring you the same things your wife does.
Your wife might think you should get everything from HER, personally. But you do get emotional support and connections to other people: your friends, your children, your family. You couldn't expect your wife to fill all of these roles at once, and similarly she can't and shouldn't fill J's. Therefore, if you can't have J in your like, you WILL miss her, but that doesn't mean you don't care dearly about your wife.
Maybe your wife needs to know you will always pick her over J, that as your wife she has a special status. That would mean she would be your primary and J would be a secondary. If you can all live with that (I'm mostly thinking about J here. Can she live with coming after your and your wife's relationship? Some people prefer it that way, some don't), then it might work very well for you.
You might want to have your wife read this thread or others on the forums and see how she feels about things, knowing that each case is of course unique, therefore what works for some of us might look terrible for her, but other things might resonate with her more.
I hope you can find a way to make your intimate network work out. It's important to deal with your wife's feelings of jealousy and not ignore them, but it's also important to know that feeling jealousy doesn't mean the situation itself is wrong. You might be jealous that a friend gets a raise when you didn't, for instance, but that in no way means they didn't deserve the raise.
When jealousy occurs, you need to work on yourself and identify the feelings. What causes the jealousy? And can we work on the underlying issue? It's possible to eliminate the jealousy while keeping the situation.
Good luck with everything!