What can I expect from breaking off with my fiance after 16 and 1/2 years together, 3 children a house and 2 cars?

1) Anything which you own jointly (e.g. a home, either/both of the cars, a joint bank account, etc.) is half yours. You have the right to be paid the proper value for your share, and if you and your fiance cannot voluntarily work out how to share, split, etc. these assets, you can go to court to enforce your rights.

2) If your children are still minors and you get custody of them, you are entitled to child support.

Unfortunately, the above may be all that you are entitled to. "Fiance" is not a relationship recognized by the law; it does  not give you any rights. Being married is a legal relationship, which conveys rights; being engaged is not and does not.

Sometimes, having lived together for many years may provide some right to support, but that is more the exception than the rule. In a very small number of states, people who have lived together for a long time while holding themself out as husband and wife (if you did this) would be considered to have a common law marriage. Since these rules vary by state, you should consult with a family law attorney to see whether they benefit you, and also to understand issues of child custody and support.